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/imagearea.cc

@@ -220,11 +220,16 @@ bool ImageArea::on_scroll_event (GdkEventScroll* event) {
 
     CropWindow* cw = getCropWindow (event->x, event->y);
     if (cw) {
-        if (event->direction==GDK_SCROLL_UP)
-            cw->zoomIn ();
-        else 
-            cw->zoomOut ();
-            return true;
+    	int newCenterX = (int)event->x;
+    	int newCenterY = (int)event->y;
+        if (event->direction==GDK_SCROLL_UP && !cw->isMaxZoom()) {
+            cw->findCenter (1, newCenterX, newCenterY);
+            cw->zoomIn (true, newCenterX, newCenterY);
+        }
+        else if (!cw->isMinZoom()) {
+            cw->findCenter (-1, newCenterX, newCenterY);
+            cw->zoomOut (true, newCenterX, newCenterY);
+        }
     }
     return true;
 }