1) -qt-xcb

a) Use xkb from the 3rd party libs. As it is done for the other xcb
dependencies when qt configure with -qt-xcb.

2) -system-xcb (default)

a) If xkb found then use xkb from the system. (Currenly xkb is not
enabled by default when configuring libxcb library).

b) If xkb can't be found on the system then keyboard state will be
updated from X11 core events.

Change-Id: I7c3dbce6daa2cec52067cd5af80f19040233a0db
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>