UI: Fix stats widget appearance on Yami themes

- QSS changes fix background color on the stats dialog.
- Making OBSBasicStats derive from QFrame fixes the background color
and round bottom corners on the stats dock.

UI/window-basic-stats.hpp

@@ -12,7 +12,7 @@
 class QGridLayout;
 class QCloseEvent;
 
-class OBSBasicStats : public QWidget {
+class OBSBasicStats : public QFrame {
 	Q_OBJECT
 
 	QLabel *fps = nullptr;