rose edit: progress-bar/hourglass for long-running custom macro

[benfitzpatrick]

Some custom macros can take a while to run in rose edit. We should implement something to entertain the user a bit while this is happening - a progress bar, hourglass, or something else.

A progress bar is probably possible if we use a separate thread for the actual macro.

[matthewrmshin]

This is related to #286 and #722.