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/guiutils.h

@@ -71,4 +71,42 @@ public:
 	}
 };
 
+class MyComboBox : public Gtk::ComboBox {
+
+	bool on_scroll_event (GdkEventScroll* event);
+
+public:
+	MyComboBox ();
+};
+
+class MyComboBoxText : public Gtk::ComboBoxText {
+
+	bool on_scroll_event (GdkEventScroll* event);
+
+public:
+	MyComboBoxText ();
+};
+
+class MySpinButton : public Gtk::SpinButton {
+
+	bool on_scroll_event (GdkEventScroll* event);
+
+public:
+	MySpinButton ();
+};
+
+class MyHScale : public Gtk::HScale {
+
+	bool on_scroll_event (GdkEventScroll* event);
+};
+
+class MyFileChooserButton : public Gtk::FileChooserButton {
+
+protected:
+	bool on_scroll_event (GdkEventScroll* event);
+
+public:
+	MyFileChooserButton (const Glib::ustring& title, Gtk::FileChooserAction action=Gtk::FILE_CHOOSER_ACTION_OPEN);
+};
+
 #endif