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/batchqueuepanel.h

@@ -22,6 +22,7 @@
 #include <gtkmm.h>
 #include <batchqueue.h>
 #include <saveformatpanel.h>
+#include <guiutils.h>
 
 class RTWindow;
 class BatchQueuePanel : public Gtk::VBox,
@@ -37,7 +38,7 @@ class BatchQueuePanel : public Gtk::VBox,
         sigc::connection stopConnection;
 
         Gtk::Entry* outdirTemplate;
-        Gtk::FileChooserButton* outdirFolder;
+        MyFileChooserButton* outdirFolder;
         Gtk::RadioButton* useTemplate;
         Gtk::RadioButton* useFolder;
         SaveFormatPanel* saveFormatPanel;