Partially hidden toolbars can now be scrolled with the mouse wheel

...thanks to the new MyScrolledToolbar class (guiutils.h/cc). This is
valid for the FileBrowser tab (first and second line can be scrolled
individually) and the top and bottom bar of the Editor(s) tab.

see #4035

rtgui/filecatalog.h

@@ -73,6 +73,7 @@ private:
 
     Gtk::HBox* buttonBar;
     Gtk::HBox* hbToolBar1;
+    MyScrolledToolbar* hbToolBar1STB;
 
     Gtk::HBox* fltrRankbox;
     Gtk::HBox* fltrLabelbox;