Extend color selector example
[GTK] add color selector backend
fix HSL conversion in DefaultColorSelectorBackend
extend DefaultColorSelectorBackend
* replace text entries with spin buttons
* add opacity slider
Merge remote-tracking branch 'mono/master' into add-color-selector-backends

Conflicts:
	Xwt.Gtk/Xwt.Gtk.csproj
Merge pull request #359 from sevoku/add-color-selector-backends

Optimize/Add color selector backends