Save language chosen for spell check for each room

Before, when a user changed the language for spell check,
that language was saved by gtk application-wide.

With this commit, the language chosen by the user for spell check gets saved
in the Matrix account data of the particular room the language was chosen in;
and, by syncronizing, it also gets saved in the new field `language` of Room.
When the user enters a room, gspell gets set for the language saved
for that room.