Win32: Fix a minor fullscreen bug.
If the window is maximized before going fullscreen, the taskbar appears on top, which is not desirable.
Also, apply the maximized state properly again after exiting, if the user had the window maximized.
Qt: Allow building with system ffmpeg. Also update b.sh to set cmake and qmake flags in case --qt is at the end of the arguments.
Merge pull request #6651 from thedax/win32-fullscreen

Win32: Fix a minor fullscreen bug.
Qt: Change target name to ppsspp by default.Install icons and desktop file on Linux. Use a different builddir than SDL by default.
Should help distros packaging ppsspp. Note: if you currently use PPSSPPQt, you will need to move the config dir from PPSSPPQt to ppsspp.
For distro packagers, you can now use PREFIX and DESTDIR for installs.