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.
Cody style nitpick: Please be consistent about braces style. We're normally using hanging braces (and you did in other places as well, except for function bodies.

So, either
```C++
if (paused) {
    // Notify UI that time is paused.
    LuaEvent::Queue("onGamePaused");
} else {
    // Notify the UI that time is running again.
    LuaEvent::Queue("onGameResumed");
}
```