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/rawprocess.cc

@@ -26,7 +26,7 @@ RawProcess::RawProcess () : Gtk::VBox(), FoldableToolPanel(this)
 {
    Gtk::HBox* hb1 = Gtk::manage (new Gtk::HBox ());
    hb1->pack_start (*Gtk::manage (new Gtk::Label ( M("TP_RAW_DMETHOD") +": ")),Gtk::PACK_SHRINK, 4);
-   dmethod = Gtk::manage (new Gtk::ComboBoxText ());
+   dmethod = Gtk::manage (new MyComboBoxText ());
    for( size_t i=0; i< procparams::RAWParams::numMethods;i++)
 	   dmethod->append_text(procparams::RAWParams::methodstring[i]);