MainFrame: fix window style misdetection

WS_OVERLAPPED is 0, detect it the roundabout way.
MainFrame: make functor operator() const
FileAssoc: resolve possible deadlock

Moral: think twice before blocking an STA thread.
MainFrame: restore window before auto-zoom

Fixes #4360
Don't show stereo icon for flash media

It doesn't indicate anything actually.
Don't use flash window size as video size

It doesn't make sense and causes problems.

Fixes #4361
Increase default flash video size