Add support for file chooser buttons

This commit adds support for file chooser buttons. They are represented
as a slice of tuples `(text, action)`. The original function uses
varargs to support an arbitrary number of arguments. This implementation
supports up to 3 buttons.

**This is a breaking change.**

See: https://developer.gnome.org/gtk3/stable/GtkFileChooserDialog.html#gtk-file-chooser-dialog-new

Improve file chooser with buttons reliability

Following the reviews in #602, this commit:
- uses the `with_buttons` function instead of modifying `new` to keep
  backward compatibility.
- saves the button texts in `Stash` objects to avoid invalid pointers
- panics when the number of buttons is too high (instead of silently
  truncating the number of buttons)

Remove doc comment for `FileChooserDialog::with_buttons`

Remove local variables in `FileChooserDialog::with_buttons`

This commit removes the Stash variables from
`FileChooserDialog::with_buttons`, as requested in the PR #602

Fix formatting issue

Keep `downcast_unchecked` method on the same line as the closing
parenthesis of the chopped-down call to `Widget::from_glib_none`.

src/file_chooser_dialog.rs

@@ -5,21 +5,83 @@
 use ffi;
 use glib::translate::*;
 use glib::object::{Downcast, IsA};
+use libc::c_char;
 use std::ptr;
 use FileChooserAction;
 use FileChooserDialog;
+use ResponseType;
 use Widget;
 use Window;
 
 impl FileChooserDialog {
-    pub fn new<T: IsA<Window>>(title: Option<&str>, parent: Option<&T>, action: FileChooserAction)
-            -> FileChooserDialog {
+    // TODO: Keep the other constructor with buttons support as the only constructor (this one was
+    //       left for compatibility) and rename it to `new` for consistency.
+    pub fn new<T: IsA<Window>>(title: Option<&str>, parent: Option<&T>, action: FileChooserAction) -> FileChooserDialog {
         assert_initialized_main_thread!();
         unsafe {
-            Widget::from_glib_none(
-                ffi::gtk_file_chooser_dialog_new(title.to_glib_none().0, parent.to_glib_none().0,
-                    action.to_glib(), ptr::null_mut()))
-                .downcast_unchecked()
+            Widget::from_glib_none(ffi::gtk_file_chooser_dialog_new(
+                title.to_glib_none().0,
+                parent.to_glib_none().0,
+                action.to_glib(),
+                ptr::null::<c_char>()
+            )).downcast_unchecked()
+        }
+    }
+
+    pub fn with_buttons<T: IsA<Window>>(title: Option<&str>, parent: Option<&T>, action: FileChooserAction, buttons: &[(&str, ResponseType)]) -> FileChooserDialog {
+        assert_initialized_main_thread!();
+        unsafe {
+            Widget::from_glib_none(match buttons.len() {
+                0 => {
+                    ffi::gtk_file_chooser_dialog_new(
+                        title.to_glib_none().0,
+                        parent.to_glib_none().0,
+                        action.to_glib(),
+                        ptr::null::<c_char>()
+                    )
+                },
+                1 => {
+                    ffi::gtk_file_chooser_dialog_new(
+                        title.to_glib_none().0,
+                        parent.to_glib_none().0,
+                        action.to_glib(),
+                        buttons[0].0.to_glib_none().0,
+                        buttons[0].1.to_glib(),
+                        ptr::null::<c_char>(),
+                    )
+                },
+                2 => {
+                    ffi::gtk_file_chooser_dialog_new(
+                        title.to_glib_none().0,
+                        parent.to_glib_none().0,
+                        action.to_glib(),
+                        buttons[0].0.to_glib_none().0,
+                        buttons[0].1.to_glib(),
+                        (buttons[1].0.to_glib_none() as Stash<*const c_char, str>).0,
+                        buttons[1].1.to_glib(),
+                        ptr::null::<c_char>(),
+                    )
+                },
+                3 => {
+                    ffi::gtk_file_chooser_dialog_new(
+                        title.to_glib_none().0,
+                        parent.to_glib_none().0,
+                        action.to_glib(),
+                        buttons[0].0.to_glib_none().0,
+                        buttons[0].1.to_glib(),
+                        (buttons[1].0.to_glib_none() as Stash<*const c_char, str>).0,
+                        buttons[1].1.to_glib(),
+                        (buttons[2].0.to_glib_none() as Stash<*const c_char, str>).0,
+                        buttons[2].1.to_glib(),
+                        ptr::null::<c_char>(),
+                    )
+                },
+                _ => {
+                    // TODO: Support arbitrary number of buttons once variadic functions are supported.
+                    //       See: https://github.com/rust-lang/rust/issues/44930
+                    panic!(format!("`FileChooserDialog::with_buttons` does not support 4+ buttons, received {}", buttons.len()))
+                }
+            }).downcast_unchecked()
         }
     }
 }