Change TalkatuAttachment to an interface and move it's implementation to TalkatuSimpleAttachment.
Also included the fix for TALKATU-82.
Testing Done:
Ran the demo in gdb with `G_DEBUG=fatal-warnings` and verified that attachments still worked as expected.
Bugs closed: TALKATU-82
Reviewed at https://reviews.imfreedom.org/r/24/