[GTK3] size request has preference over min size
add ColorPicker button with a default xwt based backend
[GTK] add color picker backend
[GTK] use system default title in SelectColorDialog, if no title is specified
Merge branch 'fix-gtk-select-color-dialog-title'
Merge branch 'add-color-picker'

Conflicts:
	TestApps/Samples/Samples/ColorSelectorSample.cs
	Xwt.Gtk/Xwt.Gtk.csproj
	Xwt.Gtk/Xwt.GtkBackend/GtkEngine.cs
	Xwt/Xwt.csproj
Merge branch 'gtk3'