Store correctly the window position with Wayland

Store correctly the window position with Wayland

To do this, hard-coded test for xcb had to be replaced with a call to
platformName(). Since this method does not exist in Qt4, we emulate
it.

Note that Qt5 uses xcb bindings for X11 system, while Qt4 relies on
older X11 bindings. We return platorm == "qt4x11" in this case.

Fixes bug #11746.

(cherry picked from commit 222a317dd243fb18f01bfa6e994902fee06ae3db)