[Gtk] fix pointer coordinates for mouse events

If a widget is a container and a mouse event is raised
by one of its children with own child Gdk.Window, the
pointer coordinates are relative to the child and not
the widget itself. In this case the pointer coordinates
should be updated.
[Gtk3] reenable GetTooltipContext for Gtk3
[Gtk3] remove useless Gtk3 check
[Gtk] add major version check (support Gtk3)
[Gtk] use version detection for toolkit features
[Gtk3] fix build on windows
 * don't provide package names in project file
[Gtk3] complete Xwt-Gdk.RGBA-Xwt conversion
[Gtk] fix image default size handling

ImageDescription.WithDefaultSize is used to fit the
image into a default size for some component like
a dialog or button. It should always return an image
with the desired default size to fit the system
design guidelines.
(78f999f follow up)
[Gtk] make GtkAlertDialog look more native

Let Gtk.MessageDialog do the layout stuff and make use
of Gtk.MessageType for a more native icon styles.
add new message dialog sample
Merge remote-tracking branch 'sevoku/fix-gtk-mouse-event-coords'
Merge remote-tracking branch 'sevoku/gtk3-fixes'

Conflicts:
	Xwt.Gtk/Xwt.GtkBackend/Conversion.cs
	Xwt.Gtk/Xwt.GtkBackend/GtkEngine.cs
Merge remote-tracking branch 'sevoku/optimize-gtk-message-dialogs'

Conflicts:
	Xwt.Gtk/Xwt.GtkBackend/Conversion.cs
	Xwt.Gtk/Xwt.GtkBackend/GtkWorkarounds.cs
Merge remote-tracking branch 'sevoku/extend-samples'