add ColorPicker button with a default xwt based backend
[GTK] add color picker backend
Merge remote-tracking branch 'mono/master' into add-color-picker

Conflicts:
	Xwt.Gtk/Xwt.Gtk.csproj
	Xwt/Xwt.csproj
Merge branch 'add-color-picker' of https://github.com/sevoku/xwt into sevoku-add-color-picker

Conflicts:
	TestApps/Samples/Samples/ColorSelectorSample.cs
	Xwt.Gtk/Xwt.Gtk.csproj
	Xwt.Gtk/Xwt.Gtk3.csproj
	Xwt.Gtk/Xwt.GtkBackend/GtkEngine.cs
Merge branch 'sevoku-add-color-picker'
[Mac] Add native implementation of ColorPicker
Add missing event mapping in ColorPicker
[Mac] Fix crash when checking if a widget has a native parent
Throw exception if event mapping is invalid