Joining all the global simulation variables in one

The global variables in engglobs.h refering to the state of the
simulation are going to be managed through a unique variable (actually a
pointer to the real variable), called 'world'. This refactoring starts
with the coding of the new type of that variable (Game_State), and its
required subtypes.

The use of a composite 'game world' or 'game state' variable is a common
technique in games, to try to minimize the use of global variables.
Substituting the old variables for the new struct

The old variables in engglobs.h/shrglobs.c, using for the simulation,
are being substituted by the new game world variable 'world'.

A new function world_init() is added For the inicialization of the
required fields of the new variable 'world'.