Add support for Wayland seat: keyboard and pointer

Rudimentary support for input events. Events from Wayland are forwarded
to X's root window using the XTest extension.

Currently supported:
* left/middle/right mouse button
* keyboard events

Not supported:
* additional mouse buttons
* mouse wheel
* touch events

Obviously this is a rather huge hack and is only intended till we have
XWayland support and proper input redirection inside KWin.