Move TalkatuHistory from a GtkTextView base to GtkListBox

Make TalkatuInput return the local time if a timestamp has not yet been set.

Add an toggle button for setting the message's edited state

Added TalkatuHistoryRow which is what will be used to display message

Add TalkatuHistoryRow to the documentation.

Add TalkatuScrolledWindow that subclasses GtkScrolledWindow but autoscrolls and has keybindings callbacks for page up and page down.

Testing Done:
Compile, docs, and manual testing in the demo. The i18n stuff is broken for unrelated reasons, which I'll fix in another review request.

Bugs closed: TALKATU-78

Reviewed at https://reviews.imfreedom.org/r/56/