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/renamedlg.cc

@@ -38,7 +38,7 @@ RenameDialog::RenameDialog (Gtk::Window* parent)
     
     Gtk::HBox* tbox = Gtk::manage (new Gtk::HBox());
     useTmpl = Gtk::manage (new Gtk::CheckButton (M("FILEBROWSER_USETEMPLATE")));
-    templates = Gtk::manage (new Gtk::ComboBox ());
+    templates = Gtk::manage (new MyComboBox ());
     templateModel = Gtk::ListStore::create (templateColumns);
     templates->set_model (templateModel);
     templates->pack_start (templateColumns.tmplName);