Added Window Closing with Key-Combination.

Added the functionality to close windows with a given key-combination.
When the key is pressed, it destorys the window from the xserver and
removes it from the managed array. Finally it then redraws the screen.

Fixes #11

Signed-off-by: Martin Jorn Rogalla <martin@martinrogalla.com>
Added SwitchTilingMode command in the switch statement
Merge pull request #22 from AirWM/AdditionalKeyBindings

Added Window Closing with Key-Combination.