application: Defer loading of room lists on metered connections

We currently load all room lists immediately on startup, to hopefully have them readily available when the user opens the join dialog. In case the user doesn't use the dialog, this means we download the lists unnecessarily, trading efficiency for convenience. This is a reasonable trade-off where bandwidth usage isn't a concern (that is, network access is sufficiently fast and cheap), but may very well be the wrong choice otherwise.

At least in case where NetworkManager tells us that a connection is metered, deferring loading the room lists until the join dialog is used is the better option.

Fixes #29 (closed)