Implement loading new resources while loading new levels

[ivanperez-keera]

After the last change to the resource manager (c90e2fb), resources are loaded only once.

This should change so that resources are loaded-unloaded as the game progresses.

A possible approach would be to:

• 1: Obtain needed resources from the current game state.
• 2: Obtain used resources from the current resource manager.
• 3: Unneeded resources (to be unloaded) is (2) - (1)
• 4: Resources to load is: (1) - (2).