Main: Replace manual window geometry handling with KConfig.WindowStateSaver
Replaces the manual window geometry logic with KConfig.WindowStateSaver
, which means there is less custom code to maintain.
PlasmaTube can currently remember if the window is maximized. KConfig.WindowStateSaver can do this as well after a recent KConfig commit, but I am not sure where the code is in PlasmaTube that handles this. I don't think I removed any existing code that remembers the maximized state, so that may need to be taken out as part of this MR.
All feedback is welcome. Thanks!