use png logo in frmabout

Fixes https://github.com/notepadqq/notepadqq/issues/51 for me. The "About" window size is fixed, so using a fixed size icon should be okay. The SVG logo was scaled to 150x150, right? -> https://github.com/notepadqq/notepadqq/blob/master/src/ui/frmabout.ui#L116