xwayland: Refactor bits of XKB functionality

[Kristian Høgsberg]

Hi list,

Here's few xkb related patches that refactor and expose xkb functionality.
There should be no change in behavior in these patches, the idea is to
expose new entry points into existing code.

This is in preparation for merging xwayland, where the X server has to
parse and apply xkb maps coming from the wayland server.  In this case we
need the ability to parse the keymap from a string and apply a keymap to
a device.

Kristian