Load and use Qt application style sheets from Plover.qss file

[MarkoSchuetz]

Is your feature request related to a problem? Please describe.

A use case would be to customize Plover plugins act/look more like OSDs. Specifically, to have the word tray window displayed with a fully customized style, e.g. transparent background (can also be achieved using window rules), a different font size and foreground color than the defaults, minimizing the padding around the word list, ....

Describe the solution you'd like

A straightforward solution is to implement looking for a Qt style sheet file in default locations, defining some sensible fallback values (e,g, the ones Plover uses currently) and setting the style sheet with the QApplication:setStyleSheet() function. In principle this would be a few lines only, but I expect the error/exception/fallback handling to make up most of the lines for this change. This solution would also provide the means for users to experiment with the style sheets on this Gallery Of Qt QSS Based Styles

Describe alternatives you've considered

I've considered KWin window rules to set e.g. the window background transparency, but that only goes part of the way.