QTfrontend: Fix flickering and bad offset of control config popup
If you opened the pop-up to change a key, there was a very short flicker at the left side
and the pop-up had a bad offset which sometimes caused entries to be out of screen bounds.
07f3bf8d98a3
Hedgewars.png had a resolution more than twice as high as required; also fixed some (mostly obsolete) code that also contained a typo that would cause a warning/note in clang
