Implemented read/write access to platform clipboard

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Fixed clipboard encoding in Xlib backend

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Fixed encoding of clipboard data in WinDOS backend

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>