Put button configuration in setting file

[Yves Rizoud]

For platforms that have "joystick button" input, the behavior is hard-coded in input.c (See the Joybutton_* ) : Which button is a click, which one is the "shift" modifier etc.
I know such settings will never be editable from within the program, but I'd like to at least put the settings in the .ini file, so that somebody who prepared a package for a specific device can fine-tune the settings.

This also applies to the "KEY_ESC" currently hard-coded in global.h

[PulkoMandy]

Move open tickets to 2.7 milestone