Pad script area line count by 1 to avoid scrolling the cursor offscreen.

Fixes ticket #721. Note that this does occasionally mean that the script area
will display a blank line with no line number (in the rare circumstance that
the old behavior that this replaces was actually the correct behavior). This
is just a quick patch job, and seems to be correct in something like 75% of
cases.

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14422 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd