Made text datablock properties 'is_in_memory' and 'is_dirty' editable in Python.

The rationale of this revision is to provide an easy way to make text datablocks internal
from within Python (i.e., by setting these properties to True and the 'filepath' property
to empty string).

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57153 954f8c5b-7b00-dc11-b283-0030488c597c