Support SDL2's mouse wheel event in mouse_handler_base.

Allows to scroll the map with the wheel.
Wrap old SDL code in a preproc directive.
Support for SDL2's mouse wheel event in the GUI1 slider.
Wrap old SDL code in a preproc directive.
Support for SDL2's mouse wheel event in the GUI1 scrollarea.
Support for SDL2's mouse wheel event in the GUI1 scrollbar.
Wrap old SDL code in a preproc directive.
Switch vertical scrolling directions.
Support for SDL2's mousw wheel event in the palette manager.
Wrap old SDL code in a preproc directive.
Make a constant variable a reference.
Support for SDL2's mouse wheel event in GUI2.
Wrap old SDL code in a preproc directive.