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/icmpanel.h

@@ -21,6 +21,7 @@
 
 #include <gtkmm.h>
 #include <adjuster.h>
+#include <guiutils.h>
 
 #include <toolpanel.h>
 
@@ -48,14 +49,14 @@ class ICMPanel : public Gtk::VBox, public AdjusterListener, public FoldableToolP
         Gtk::RadioButton*  ifromfile;
         Gtk::CheckButton*  igamma;
         Gtk::CheckButton*  ckbBlendCMSMatrix;
-        Gtk::ComboBoxText* wnames;
-        Gtk::ComboBoxText* wgamma;
+        MyComboBoxText*    wnames;
+        MyComboBoxText*    wgamma;
 		
-        Gtk::ComboBoxText* onames;
+        MyComboBoxText*    onames;
         Gtk::RadioButton*  ofromdir;
         Gtk::RadioButton*  ofromfile;
         Gtk::RadioButton*  iunchanged;
-        Gtk::FileChooserButton* ipDialog;
+        MyFileChooserButton* ipDialog;
         Gtk::RadioButton::Group opts;
         Gtk::Button*        saveRef;
         sigc::connection   ipc;