Text data block were not considered special in the recursive purge
function. So they would get deleted if they had no actual users.
To fix this we instead make text data block use "fake user" so that
addon authors can specify script files that should be removed if nothing
is using it anymore.
Per default, new text object have "fake user" set. So functionality
wise, the user has to explicitly specify that they want the text object
to be purge-able.
Reviewed By: Bastien
Differential Revision: http://developer.blender.org/D10983
756c9b2219