First steps towards configurable mouse input. Soon, this new code

First steps towards configurable mouse input.  Soon, this new code
(mainly in src/Input/input.cxx) will make src/GUI/mouse.cxx obsolete
and bring the mouse into the same input system as the joystick and
keyboard.  This is just preliminary work allowing, covering mouse
clicks (no motion yet), and it actually crashes on a middle or right
click.

The new mouse support is disabled by default until it become stable;
to try it out, you need to configure --with-new-mouse.