Add cursor position and selection support to TextEntry

* add properties for cursor position and selection bounds
* add SelectedText property (which is writeble to support direct selection changes)
* add SelectionChanged event
* includes ComboBoxTextEntry support

WPF and GTK support is included. Mac has to be added.
Add text selection to samples
Merge remote-tracking branch 'origin/add-text-cursor-selection' into gtk3
[GTK] move toolkit (gtk2/3) specific backend code to separate files
[GTK3] add Widget Opacity support