Fix compilation warnings

[NathanBnm]

There are some Warnings on building from source, you have to create a header file in order to declare the g_signal_emit_by_name function and add this header to the meson.build configuration and fix deprecation with Gtk.Screen.

../src/keycapture.c: In function ‘key_pressed_cb’:
../src/keycapture.c:43:13: warning: implicit declaration of function ‘g_signal_emit_by_name’ [-Wimplicit-function-declaration]
             g_signal_emit_by_name (arg, "captured", XKeysymToString (XkbKeycodeToKeysym (dpy, detail, 0, shiftlevel)), FALSE);
             ^~~~~~~~~~~~~~~~~~~~~
[4/14] Compiling Vala source ../src/Interfaces/Media...ainWindow.vala ../src/Settings.vala src/Config.vala.
../src/Interfaces/MediaKeys.vala:31.9-31.48: warning: DBus methods are recommended to throw at least `GLib.Error' or `GLib.DBusError, GLib.IOError'
        public abstract void GrabMediaPlayerKeys (string application, uint32 time) throws IOError;
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
../src/Interfaces/MediaKeys.vala:32.9-32.51: warning: DBus methods are recommended to throw at least `GLib.Error' or `GLib.DBusError, GLib.IOError'
        public abstract void ReleaseMediaPlayerKeys (string application) throws IOError;
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
../src/Widgets/KeyView.vala:225.13-225.36: warning: Gtk.Window.set_has_resize_grip has been deprecated since 3.14
../src/Widgets/SelectionArea.vala:32.13-32.36: warning: Gtk.Window.set_has_resize_grip has been deprecated since 3.14
../src/Widgets/SelectionArea.vala:60.36-60.51: warning: Gdk.Screen.width has been deprecated since 3.22
../src/Widgets/SelectionArea.vala:61.37-61.53: warning: Gdk.Screen.height has been deprecated since 3.22
../src/Widgets/SelectionArea.vala:160.18-160.35: warning: Gtk.Widget.is_composited has been deprecated since 3.22
../src/MainWindow.vala:82.18-82.35: warning: Gtk.Widget.is_composited has been deprecated since 3.22
../src/MainWindow.vala:96.33-96.53: warning: Gdk.Screen.get_n_monitors has been deprecated since 3.22
../src/MainWindow.vala:103.17-103.37: warning: Gdk.Screen.get_n_monitors has been deprecated since 3.22
../src/MainWindow.vala:107.27-107.52: warning: Gdk.Screen.get_primary_monitor has been deprecated since 3.22
../src/MainWindow.vala:108.21-108.51: warning: Gdk.Screen.get_monitor_scale_factor has been deprecated since 3.22
../src/MainWindow.vala:110.60-110.75: warning: Gdk.Screen.get_width has been deprecated since 3.22
../src/MainWindow.vala:115.61-115.77: warning: Gdk.Screen.get_height has been deprecated since 3.22
../src/Widgets/Countdown.vala:37.13-37.27: warning: Gtk.Widget.margin_left has been deprecated since 3.12
../src/Widgets/Countdown.vala:37.31-37.46: warning: Gtk.Widget.margin_right has been deprecated since 3.12
../src/MainWindow.vala:514.13-514.32: warning: Gtk.Dialog.get_action_area has been deprecated since 3.12
../src/MainWindow.vala:522.13-522.32: warning: Gtk.Dialog.get_action_area has been deprecated since 3.12
../src/MainWindow.vala:262.31-262.50: warning: Gtk.Dialog.get_action_area has been deprecated since 3.12
../src/MainWindow.vala:279.21-279.52: warning: Gdk.Screen.get_monitor_geometry has been deprecated since 3.22
../src/MainWindow.vala:280.34-280.64: warning: Gdk.Screen.get_monitor_scale_factor has been deprecated since 3.22
../src/MainWindow.vala:288.13-288.44: warning: Gdk.Screen.get_monitor_geometry has been deprecated since 3.22
../src/MainWindow.vala:289.21-289.51: warning: Gdk.Screen.get_monitor_scale_factor has been deprecated since 3.22
../src/MainWindow.vala:356.25-356.64: warning: Gdk.Screen.get_n_monitors has been deprecated since 3.22
Compilation succeeded - 24 warning(s)