The checkboxes were not settable in the previous implementation.

The checkboxes were not settable in the previous implementation.

The problem seems to stem from the old
  flags="dont-build-content"
setting, which selects tree vs. content templating.  See
  https://developer.mozilla.org/en/XUL_Tutorial/Trees_and_Templates
  https://developer.mozilla.org/en/XUL/Attribute/flags
for details.

Also hooked checkbox handler (checkbox_activity()) on as a method of
the tree.view, which makes it easier to only act on checkbox activity,
and locate the active row.

Other tidbits:
  * Set type="application/javascript" in dirtag.js import.
    Might as well to that formally.
  * selstype="primary" in checkbox-tree.
    I'd rather disable selection entirely, but this at least keeps it off the
    checkbox column.