Expose game paused to Lua; disable bulletin board when paused

Closes #2732.

Needed to add a signal to notify when widgets are visible/invisible. This is used to ensure we lock/unlock adverts when they're opened in chat dialogs.

Game pause/resume events are sent by the Game object on construction (to provide the initial state) and on relevant changes to time accel.
I think this should say `Locked adverts are not removed by RemoveAdvert until they are unlocked.`