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/