Solving issue 735 : "Zooming with mouse wheel should zoom where the mouse is".

The scroll wheel is now used for scrolling the editor's tab content. If you want
to change the spin button's/slider's value or combobox entry, press the SHIFT key
while using the scroll wheel.

This patch also:
- reduce the spin buttons size (50px wide, instead of 70px)
- reduce the minimum size of the combobox (tool panels can now be narrower before
  showing the horizontal scrollbar)

Note to developers:
-------------------
When creating GUI for editor tools, please use the MyComboBox, MyComboBoxText,
MySpinButton and MyFileChooserButton derived class only in order to handle
the scroll wheel properly (-> #include <guiutils.h>).

Known bug: The MyFileChooserButton thas is a FILE_CHOOSER_ACTION_SELECT_FOLDER
does not respond correctly to the new behaviour.

rtgui/adjuster.h

@@ -21,6 +21,7 @@
 
 #include <gtkmm.h>
 #include <editedstate.h>
+#include <guiutils.h>
 
 class Adjuster;
 class AdjusterListener {
@@ -35,8 +36,8 @@ class Adjuster : public Gtk::VBox {
   protected:
     Gtk::HBox* hbox;
     Gtk::Label* label;
-    Gtk::HScale* slider;
-    Gtk::SpinButton* spin;
+    MyHScale* slider;
+    MySpinButton* spin;
     Gtk::Button* reset;
     AdjusterListener* adjusterListener;
     sigc::connection delayConnection;