initial version of a P/M mouse callback
- always use page 6 for P/M mouse cursor
- make cursor character of text mode callback configurable
- change default cursor character of text mode callback from 'plus' to 'diamond'
- set P/M callback as default
address issue raised in the pull request; add support to not use page 6
again
Selection whether to use page 6 for mouse P/M data is not done in
Makefile.inc, like the other compile-time options. Small fix in the
P/M mouse "show" routine: adapt mouse cursor colors to current
screen colors.
Merge pull request #106 from groessler/something_to_pull

add P/M (aka 'Sprite') mouse callback for Atari