Add groupbox borders, reorder Base stylesheet (#810)

1. According to https://doc.qt.io/qt-6/stylesheet-reference.html, paintEvent(QPaintEvent *) function should be implemented for custom class derived from QWidget

For this I added QssWidget class which is now base for CollapsibleGroupBox and StaticGroupBox. Now stylesheets for these classes work as expected.

2. Some QGroupBox used in GUI are replaced with StaticGroupBox.

3. CollapsibleGroupBox and StaticGroupBox got borders.

4. To ease the navigation and understanding of Base.stylesheet, I grouped settings by widget type.

Resolves #810 (closed)

GUI/View/Widget/GroupBoxes.h

@@ -15,8 +15,7 @@
 #ifndef BORNAGAIN_GUI_VIEW_WIDGET_GROUPBOXES_H
 #define BORNAGAIN_GUI_VIEW_WIDGET_GROUPBOXES_H
 
-#include <QGroupBox>
-#include <QObject>
+#include <QWidget>
 
 class QAction;
 class QHBoxLayout;