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'm not sure about this one (not very familiar with this part of the code base): Does becoming invisible always mean "closed"? Or can it also mean, that it's totally covered by another widget and becomes visible again, when the covering widget will be closed?