also expose it through the QXcbScreenFunctions platform header and use
it in the qsystemtrayicon_x11.cpp. This gives us static typechecking.

Change-Id: Ia9e2e2563ed5994be0b19d7bcf5a6aca92b1b760
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>