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/darkframe.h

@@ -22,6 +22,7 @@
 #include <gtkmm.h>
 #include <toolpanel.h>
 #include <rawimage.h>
+#include <guiutils.h>
 
 class DFProvider {
   public:
@@ -33,8 +34,7 @@ class DarkFrame : public Gtk::VBox, public FoldableToolPanel {
 
 protected:
 
-	Gtk::ComboBoxText* darkFrameMethod;
-	Gtk::FileChooserButton *darkFrameFile;
+	MyFileChooserButton *darkFrameFile;
     Gtk::HBox *hbdf;
     Gtk::Button *btnReset;
     Gtk::Label *dfLabel;