Add text cursor selection

This pull request adds cursor and selection handling to TextEntry.
Extends TextEntry API by:
* Properties: CursorPosition, SelectionStart, SelectionLength, SelectedText
* Events: OnSelectionChanged

Implemented in Backends: WPF, GTK (/GTK3). TODO: Mac

Gtk is a little bit dirty, since Gtk has no signals for selection changes or cursor movements with the mouse. The backend implementation handles some mouse events and checks the changes.

WPF ComboBox support is a little workaround through the WPF templating system, because ComboBox hides the inner TextBox.