Restructure crate layout
Beside the actor being separated, the Queryable
Message model is moved to crate::store
because it's
related to the database.
Now it should be possible to have all db-related code
in store
, qt/qml-model-related code in model
, and
the actors and their events and such in actor
.
The purpose of this exercise is an attempt to keep the files smaller as new models get implemented.