Use more reliable multiple select instead of checkboxes.

Use more reliable multiple select instead of checkboxes.

I couldn't figure out how to use setCellValue to change and display
the checkbox values.  The current implementation adds a "Set Tags"
button which reads off the selected rows and uploads any tag changes.

Problem with the current implementation:

When the tree is rebuilt following a data refresh, the rebuild seems
to unselect any repainted rows.  I'm not sure why.  The current
workaround is to run setup_checkboxes() some reasonable delay after
refreshing the data.  Too bad I can't find a blocking refresh, or
better yet, a post-refresh hook to use instead of setTimeout().

Anyhow, any tag selection activity by the user that happens during the
timeout will, obviously, be overwritten by the setup_checkboxes()
call.