Move gamemap initialization in mp::create to a new function.

The new function takes care of failed gamemap initialization.
Extract level set up code in mp::create and move it to new function.

New function is responsible for setting up data in 'parameters_' for the
next level.
Move level sides set up to a new function.
Move map drawing to a new function.
Minor clean-up and polish of process_event() inside mp::create.