Adapt to libcmatrix API changes

libcmatrix now stores events in its own store, and thus chatty only have to care about properly showing them.

As a side effect, the messages already stored in chatty-history shall not be shown, but since chatty matrix was considered experimental, I believe that shouldn't be much a problem (as the non-encrypted events can be loaded from server again as needed)

Please note that there could be changes to matrix database in this MR often. Changes to db shall be announced in this MR if it happens, and if db changes, you shall have to remove your old matrix.db file and run the new chatty code (the db changes made here are often backward incompatible within the context of this MR)

Please backup your ~/.purple directory before testing.

/cc @guido.gunther