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/guiutils.h

@@ -293,6 +293,18 @@ public:
     MyScrolledWindow();
 };
 
+/**
+ * @brief subclass of Gtk::ScrolledWindow in order to handle the large toolbars (wider than available space)
+ */
+class MyScrolledToolbar : public Gtk::ScrolledWindow
+{
+
+    bool on_scroll_event (GdkEventScroll* event);
+
+public:
+    MyScrolledToolbar();
+};
+
 /**
  * @brief subclass of Gtk::ComboBox in order to handle the scrollwheel
  */