(MESS) ui.c: added shortcut commands for Tape Start and Tape Stop,
by default mapped to F2 and Shift+F2, respectively. This is not meant 
as a replacement for the internal Tape Control menu, but it shall make
simpler the usage of game tapes in home computers. Notice that 
REC/RWD/FWD have still to be pressed from the internal UI and that 
if you mount multiple cassettes in a driver with multiple tape drives 
only the first one will be controllable with these shortcuts. [Fabio Priuli]

The idea is that more complex actions (e.g. save you own BASIC program 
or copy programs from a tape to another, etc.) should be performed through
the internal menu, while people just aiming to play games can use the
shortcut. 
This commit requires probably a clean build.