Removes useless includes.
Pass the images from one level to the next.

bear::engine::level_globals accept a temporary resource source from which it
will copy the image resources until being frozen. This way, we can pass images
from one level to the next and reduce the loading time.
The sounds are copied from one level to the other.

bear::engine::level_globals uses the shared and temporary resources to find the
existing sounds.
The sounds are loaded in a thread.

Half the time spent loading a level is in the loading of the level's music. This
commit moves the loading of the sounds in a thread in order to allow continuing
the process without blocking.

If one tries to play the sound before the loading completes, then the thread is
forced to finish.
Remove profiling stuff from the level_loader.