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/tonecurve.h

@@ -25,13 +25,14 @@
 #include <curveeditor.h>
 #include <curveeditorgroup.h>
 #include <mycurve.h>
+#include <guiutils.h>
 
 class ToneCurve : public Gtk::VBox, public AdjusterListener, public FoldableToolPanel, public rtengine::AutoExpListener, public CurveListener {
 
   protected:
     Gtk::HBox* abox;
     Gtk::ToggleButton* autolevels;
-    Gtk::SpinButton* sclip;
+    MySpinButton* sclip;
     Adjuster* expcomp;
     Adjuster* brightness;
     Adjuster* black;