Fix: MPC-HC did not start when using the INI file.

574c87b9f9affaae2d971c53f61b3fa803c18652 introduced an infinite-loop when writing the INI file. Registry storage was unaffected.

Fixes #4330.
MainFrame: make bExtOnTop visible to class
Don't drop out of fullscreen when loosing focus

Instead, unset the "on top" flag.
MainFrame: fix window style misdetection

WS_OVERLAPPED is 0, detect it the roundabout way.