Change the term_mouse interface a little so that it gets passed

Change the term_mouse interface a little so that it gets passed
both the raw and the cooked mouse button, with the mapping being done in
advance by the front-end.  This is useful because it allows the front-end to
use information other than the raw button (e.g. the modifier state) to decide
which cooked button to generate.
.
Front ends other than the Mac one are untested, but they just call
translate_button() themselves and pass the result to term_mouse().

git-svn-id: svn://svn.tartarus.org/sgt/putty@2721 cda61777-01e9-0310-a592-d414129be87e