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/flatfield.h

@@ -23,6 +23,7 @@
 #include <adjuster.h>
 #include <toolpanel.h>
 #include <rawimage.h>
+#include <guiutils.h>
 
 class FFProvider {
   public:
@@ -34,13 +35,13 @@ class FlatField : public Gtk::VBox, public AdjusterListener, public FoldableTool
 
 protected:
 
-	Gtk::FileChooserButton *flatFieldFile;
+	MyFileChooserButton *flatFieldFile;
 	Gtk::Label *ffLabel;
 	Gtk::Label *ffInfo;
 	Gtk::Button *flatFieldFileReset;
 	Gtk::CheckButton* flatFieldAutoSelect;
 	Adjuster* flatFieldBlurRadius;
-	Gtk::ComboBoxText* flatFieldBlurType;
+	MyComboBoxText* flatFieldBlurType;
 	Gtk::HBox *hbff;
 	bool ffChanged;
 	bool lastFFAutoSelect;