bugfix: Sometimes mouse codes come in a string, which must be unpacked.

I ran into a terminal where the mouse codes were sent as characters,
rather than numerals. In other words, instead of sending the string
"32", it would send "32".chr.  So I now check if the value is
acceptable; if not, I unpack it.

	modified:   src/antsy.rb
bugfix: sanitize cursor position on mouse highlighting

Jedi was crashing when using the mouse to highlight text, but putting
the mouse cursor beyond the last line.  So I now sanitize the cursor
position before setting text-selection marks.

	modified:   src/filebuffer.rb
	modified:   src/keymap.rb