Adding a progressbar to the loading screen

See issue #136

#### Working

* [x] show a text of what happens during loading (e.g. 'load buildings', 'load map')

#### TODO

* [ ] add a progressbar


For the progressbar we need a way to quantify the loading progress. I'm thinking of using names for the breakpoints (instead of user-centric strings like now), and let the loadingscreen handle the display of appropriate messages.
These same breakpoints can be used to run some profiling against the loading process and help calibrate the progressbar.

```python
LoadingProgress.broadcast(self, "world_init")
LoadingProgress.broadcast(self, "world_load_map")
```