Fix issues when sending images to some clients.

Limiting improves readability on wide monitors.

The immediate update of the settings is done by just exposing settings
as properties and then setting the settings as a context property.

Messages on /sync don't have a room_id

(This is such a hack and will probably break, but it works for now for
most cases...)

This is necessary to support having a picker within QML.
Eventually, this should replace the existing widget-based one.