Utilize libxkbcommon API for the keyboard backend on X11

libxkbcommon is a keymap compiler [1] which utilizes xkb
keyboard configuration database [2].

xkbcommon simplifies keyboard state handling by providing xkb_state
object which holds the current state of all modifiers, groups, LEDs,
etc, relating to the provided keymap.

Detecting modifier mapping should become simpler once it gets supported
in libxkbcommon. Also with xinput2 we could get rid of the XkbStateNotify
events, because xinput2 key press/release already include all the mod/group
info.

[1] http://xkbcommon.org/
[2] http://www.freedesktop.org/wiki/Software/XKeyboardConfig



This patch:

- Removes a dependency to -lxcb-keysyms
- Removes a dependency to XLib call - XLookupString
- Enables required functionality to fix QTBUG-27681

Task-number: QTBUG-27680
Change-Id: I10e10f873821ee02f6df72238e215a541150f38f
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>