Add keyboard selection + cut/copy support

Shift+Left/Right arrow now selects text
Shift+Ctrl+Left/Right arrow now selects words (Windows mode)
Alt+Shift+F/Alt+Shift+B now select words (Emacs mode)

Del/Backspace deletes highlighted region
Ctrl+X cuts to system clipboard (Windows mode)
Shift+Ctrl+C copys selection or full line to system clipboard (Windws mode)

New unbound function KillRegion which kills text b/w cursor and mark (unbound)
Update changes.txt