Clear/Load screen immediately after Room select

[bunnie]

Currently - when a new Room is selected - the Posts from the old Room (dialogue) are displayed until the Posts from the new Room show up. This is a really poor user experience.

Once a new room is selected, the old posts should be cleared immediately - and any posts stored in the pddb displayed ASAP - while the UI waits for Posts to arrive from the server.

There probably needs to be a call to chat::dialogue_set() immediately after the call to room_modal completes.