Improve input support for XCB

by chr for X.Org

As XCB implements only an incomplete subset of keyboard functions, its employment by application programmers is somewhat hindered. This is even amplified by the fact that complete XKB protocol descriptions have been added in 2009, but libxcb still lacks support due to missing code generators. In the course of this project the XCB keyboard support should be extended to enable XKB-unaware, XKB-capable or even XKB-aware applications.