Add support for Clipboard Virtual Channel Extension

[untoldone]

Support for copy + paste would be nice. e.g. https://msdn.microsoft.com/en-us/library/cc241066.aspx. Sample implementations can be found at:

• https://github.com/rdesktop/rdesktop/blob/master/cliprdr.c
• https://github.com/FreeRDP/FreeRDP/tree/master/channels/cliprdr

@citronneur any quick pointers if I give implementing this a shot?