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/saveformatpanel.cc

@@ -35,7 +35,7 @@ SaveFormatPanel::SaveFormatPanel () : listener (NULL) {
     Gtk::HBox* hb1 = Gtk::manage (new Gtk::HBox ());
     Gtk::Label* flab = Gtk::manage (new Gtk::Label (M("SAVEDLG_FILEFORMAT")+":"));
     hb1->pack_start (*flab, Gtk::PACK_SHRINK,4);
-    format = Gtk::manage (new Gtk::ComboBoxText ());
+    format = Gtk::manage (new MyComboBoxText ());
     format->append_text ("JPEG (8 bit)");
     format->append_text ("TIFF (8 bit)");
     format->append_text ("TIFF (16 bit)");