On Windows, use:
- QtFileDialog if FG was built with Qt support;
- PUIFileDialog otherwise.
Behavior on other platforms is unchanged. This change is motivated by
the fact that some Windows users have reported[1][2] weird,
non-deterministic behavior of WindowsFileDialog and unfortunately, no
one seems to be willing and able to fix the problem. The Qt
implementation comes for free and should be quite robust. Of course, if
someone wants to maintain the WindowsFileDialog class again, the change
can be reverted.
See discussion at [3].
[1] https://forum.flightgear.org/viewtopic.php?f=25&t=31945
[2] https://sourceforge.net/p/flightgear/mailman/message/35761650/
[3] https://sourceforge.net/p/flightgear/mailman/message/35759819/
By Gijs with additions by James. Menubar is disabled at present
since it's not usable and potentially inferior to other solutions,
but committing the code so it doesn't rot.
