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/darkframe.cc

@@ -28,7 +28,7 @@ using namespace rtengine::procparams;
 DarkFrame::DarkFrame () : Gtk::VBox(), FoldableToolPanel(this)
 {
 	hbdf = Gtk::manage(new Gtk::HBox());
-	darkFrameFile = Gtk::manage(new Gtk::FileChooserButton(M("TP_DARKFRAME_LABEL"), Gtk::FILE_CHOOSER_ACTION_OPEN));
+	darkFrameFile = Gtk::manage(new MyFileChooserButton(M("TP_DARKFRAME_LABEL"), Gtk::FILE_CHOOSER_ACTION_OPEN));
 	dfLabel = Gtk::manage(new Gtk::Label(M("GENERAL_FILE")));
 	btnReset = Gtk::manage(new Gtk::Button());
 	btnReset->set_image (*Gtk::manage(new Gtk::Image (Gtk::StockID("gtk-cancel"), Gtk::ICON_SIZE_BUTTON)));