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/exifpanel.cc

@@ -18,6 +18,7 @@
  */
 #include <exifpanel.h>
 #include <safegtk.h>
+#include <guiutils.h>
 
 using namespace rtengine;
 using namespace rtengine::procparams;
@@ -26,7 +27,7 @@ extern Glib::ustring argv0;
 
 ExifPanel::ExifPanel () : idata(NULL) {
 
-    recursiveOp = true;
+   recursiveOp = true;
 
    exifTree = Gtk::manage(new Gtk::TreeView());
    scrolledWindow = Gtk::manage(new Gtk::ScrolledWindow());
@@ -366,7 +367,7 @@ void ExifPanel::addPressed () {
     Gtk::HBox* hb2 = new Gtk::HBox ();
     
     Gtk::Label* tlabel = new Gtk::Label (M("EXIFPANEL_ADDTAGDLG_SELECTTAG")+":");
-    Gtk::ComboBoxText* tcombo = new Gtk::ComboBoxText ();
+    MyComboBoxText* tcombo = new MyComboBoxText ();
     
     tcombo->append_text ("Artist");
     tcombo->append_text ("Copyright");