[Qt-desktop] Make OpenGL always draw something, fix dialog crash.

Make OpenGL always draw something. Where there is no game, display the PPSSPP logo screen. Require ui_atlas in assets/ .
Add hack for X11 user which can have a crash when opening the first dialog and GL is drawing. The only way I found is to lock 100ms the GL display while the popup is shown.
Remove the "open file" dialog automatic display on startup, very annoying.