(This also modifies image-source, obs-text, text-freetype2, and UI)

This improves source definition versioning.  To do this, it now stores
two identifier names.  One "unversioned" which is the original name, and
one "versioned" with the version number appended.

This fixes both backward compatibility with older OBS versions, and
fixes the inability to use "add existing" in OBS itself on sources
created from older version definitions.