Appearance updates (currently bsim only)
Removed empty footer from browser check alert.
Added transient alerts to the top of the window.
Disable simulation buttons when they won't work.
The Checkoff button is only available when checkoffs are possible.
BSim is actually version 2, not 3.
Cmd-S works again, replaced icon-hdd with "Save" text.
Merge branch 'master' of github-mattpf:6004x/6.004x