This involved a refactoring to move all the media handling functions
(downloading, opening externally, etc) out of the Event delegates and
into the EventList, which manage keybinds instead.
This should also be better for performance since all these functions are
no longer duplicated for every Event in view.
Other user-noticable change: clicking on non-image media will
always download and open them no matter if the room is encrypted or not,
instead of opening non-encrypted files in browser like before. It will
be possible to still do that with an "open externally" command later.
