Streamline -geometry command-line option support

Streamline -geometry command-line option support

Instead of baking our own support for -geometry command line option on
Windows (Qt only does that for Xcb), use the fact that Qt 5 provides
the equivalent -qwindowgeometry flag.

All we need to do is to replace the -geometry option with -qwindowgeometry.

This is part of the effort in ticket #13086 to get rid of the
compile-time QPA_XCB define.