Main: Store window geometry with KConfig.WindowStateSaver

This MR implements storing the window size and maximization state using KConfig.WindowStateSaver when KRecorder is closed. It also fixes a problem found as a result of doing this, whereby the "Record a new recording" page would be displayed overtop the recordings list if KRecorder was opened in a non-widescreen configuration, by setting the DefaultPage visibility to depend on the applicationWindow().sideWidescreen property. This is possible now that KRecorder could be closed in a state where it is not widescreen, and opened again in that state, whereas before it would have opened with its default geometry.