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/hlrec.cc

@@ -28,7 +28,7 @@ HLRecovery::HLRecovery () : Gtk::VBox(), FoldableToolPanel(this) {
     enabled->set_active (false);
     pack_start (*enabled);
 
-	method = Gtk::manage (new Gtk::ComboBoxText ());
+	method = Gtk::manage (new MyComboBoxText ());
 	method->append_text (M("TP_HLREC_LUMINANCE"));
 	method->append_text (M("TP_HLREC_CIELAB"));
 	method->append_text (M("TP_HLREC_COLOR"));