Add tooltip parameter to the add_button function

The tooltip needs to display the file's path, but will be unavailable if
the label is passed as the basename only. If the tooltip value is null,
default to the label input.

dragon.c

@@ -159,7 +159,7 @@ void add_uri(char *uri) {
     }
 }
 
-GtkButton *add_button(char *label, struct draggable_thing *dragdata, int type) {
+GtkButton *add_button(char *label, struct draggable_thing *dragdata, int type, char *tooltip) {
     GtkWidget *button;
 
     if (icons_only) {
@@ -168,8 +168,9 @@ GtkButton *add_button(char *label, struct draggable_thing *dragdata, int type) {
         button = gtk_button_new_with_label(label);
     }
 
-    // Show a tooltip with the filename. Should perhaps show the full path.
-    gtk_widget_set_tooltip_text(button, label);
+    // Show a tooltip based on `tooltip` or the button's intended label.
+    // Ideally, the value is the file's full path.
+    gtk_widget_set_tooltip_text(button, tooltip != NULL ? tooltip : label);
 
     GtkTargetList *targetlist = gtk_drag_source_get_target_list(GTK_WIDGET(button));
     if (targetlist)
@@ -220,14 +221,8 @@ void add_file_button(GFile *file) {
     // be either the basename or the full path. However, the full path is
     // necessary to get the pixel buffer for image as well as for the button's
     // tooltip.
-    char *filename;
     char *path = g_file_get_path(file);
-
-    if (filename_only) {
-      filename = g_path_get_basename(path);
-    } else {
-      filename = g_file_get_path(file);
-    }
+    char *filename = filename_only ? g_path_get_basename(path) : g_file_get_path(file);
 
     if(!g_file_query_exists(file, NULL)) {
         fprintf(stderr, "The file `%s' does not exist.\n",
@@ -243,7 +238,7 @@ void add_file_button(GFile *file) {
     dragdata->text = filename;
     dragdata->uri = uri;
 
-    GtkButton *button = add_button(filename, dragdata, TARGET_TYPE_URI);
+    GtkButton *button = add_button(filename, dragdata, TARGET_TYPE_URI, path);
     GdkPixbuf *pb = gdk_pixbuf_new_from_file_at_size(path, thumb_size, thumb_size, NULL);
     if (pb) {
         GtkWidget *image = gtk_image_new_from_pixbuf(pb);
@@ -289,7 +284,7 @@ void add_uri_button(char *uri) {
     struct draggable_thing *dragdata = malloc(sizeof(struct draggable_thing));
     dragdata->text = uri;
     dragdata->uri = uri;
-    GtkButton *button = add_button(uri, dragdata, TARGET_TYPE_URI);
+    GtkButton *button = add_button(uri, dragdata, TARGET_TYPE_URI, NULL);
     left_align_button(button);
 }