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 pull request #334 from sevoku/add-text-cursor-selection

Add text cursor selection