New Tag -- select should not have tagset tags

When creating a new tag, the parent_id select box includes a list of all tags – including those that are children of tagsets. Tagsets are limited to two levels deep, and thus their children should not be available as parents.

CRM/Admin/Form/Tags.php
line 82, change to:
$allTag = array ('' => '- ' . ts('select') . ' -') + CRM_Core_BAO_Tag::getTags( );

this almost fixes it – it limits the list to only non-tagset tags. But it also excludes the tagsets themselves, which you could potentially want to add to.
could use getTagSet() and append those values

also – the main tag display only allows navigation three levels deep. but when viewing tags in the contact summary-edit page, you can view deeper levels. do we want to limit the parent options to two levels to avoid four-deep tags that have limited accessibility? (real fix would be to improve the tags tab)