better tracking of script completion

should prevent the script’s stop() method from being callable
repeatedly via cmd-period
export sessions now handle cancellation more gracefully

cancelling an export will only truncate the video now rather than
producing an incomplete, corrupt file
bumped required os version
dropped a bunch of inapplicable menu items

text styling, spell checker, etc.
overstructured
moved zoom & progress widgets to hideable status bar
added support for Responsive Scrolling™

necessitated making the container view opaque and removing the
scrollwheel/wheeldelta event data that had been exposed to the script

also removed tags display from export dialogs and moved the progress
spinner into the output textview
coming attractions
brevity, valor, etc.