Slight improvement to cursor blink timing: since the cursor doesn't

Slight improvement to cursor blink timing: since the cursor doesn't
blink when the window doesn't have focus, we don't schedule blink
timers at that point either.

Infrastructure change: term->has_focus should now not be written
directly from outside terminal.c. Instead, use the function
term_set_focus, which will sort out the blink timers as well.

git-svn-id: svn://svn.tartarus.org/sgt/putty@4911 cda61777-01e9-0310-a592-d414129be87e