Expose game paused to Lua; disable bulletin board when paused

Work towards closing #2732.

One remaining task is to give some visual cue to the user that the
bulletin board is no longer clickable.

Another could be to wrap the 'disabled on paused' semantic into
something more reusable for other UI. I think this is out of scope for
Suggest to put parenthesis around the condition for easier readability:
```C++
 bool emitPaused = (t == TIMEACCEL_PAUSED && t != m_timeAccel);
 bool emitResumed = (m_timeAccel == TIMEACCEL_PAUSED && t != TIMEACCEL_PAUSED);
```
Also, if I read the code correctly, ```SetTimeAccel()``` could be called without changing the state (see for example the call in ```Game::UpdateTimeAccel()```). I think we would only want to trigger the events if a transition between paused/unpaused happens, not again and again on every call.