[Qt4] Wrong encoding of non-ASCII strings in the title of the filepicker

[eumagga0x2a]

While investigating the circumstances of http://avidemux.org/smif/index.php/topic,17008.0.html, I played with a Qt4 build of Avidemux and saw that German umlauts like in "Videodatei wählen..." for avidemux/common/gui_main.cpp:340 in the title bar of the filepicker are displayed as garbage. This problem doesn't exist on Qt5.

Does Qt4 need some special utf-8 treatment?

[mean]

Qt4 is being obsoleted (in general, not by me)
Not sure it's worth the time