Prefer QPA implementation in qsystemtrayicon_x11 if available

In order to have the possibility to provide a custom QSystemTrayIcon
implementation in the platform theme instead of the X11 xembed based
one, the qpa implementation needs to be called. This was not possible
as qpa and x11 implementation were compile time mutual exclusive.

This change moves the qpa implementation in the shared part and the
methods in qsystemtrayicon_qpa just delegate to them. In addition the
_x11 part tries to create a QPlatformSystemTrayIcon through the
platform theme and if that succeeds the implementation prefers the qpa
variant and delegates to the same methods.

Change-Id: I6b33acac63524a77ebdce39af6eb74666f8c7561
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>