Decouple world generation and resource loading

[mofr]

Currently, resources required to build the world are loaded lazily during the act building process (WorldBuilder, LevelBuilder classes).

The goal is to split the process into two steps:

• Load all required resources (ds1, dt1)
• Build the world map

DT1 loading is also should be split into two phases:

• Loading meta-information (like collision flags etc) which is required to build the world map
• Loading textures which are required to render the visible tiles