engine/terrain: added terrain_object for immobile object handling
engine/terrain: added texture to terrain_object
engine/terrain: added drawing support for terrain_objects
engine/terrain: added terrain_object to makelist
engine/terrain: makeing it compile

damn u, circular dependencys
engine/terrain: rightclick now uses the terrain_object
engine/terrain: no more overlapping buildings

terrain_objects now can only be added, if they don't overlap/fly in
nothingness. For that last thing, i used errorcatching which probably is
an evil thing to do.
engine/terrain: removing of buildings works now.

special about this: u just have to hit the building, not the hotspot
doc: where to build docks
engine/terrain: fixed segfault, improved documentation
engine/terrain: first try of building ordering
engine/terrain: redid building drawing odering and removed some output
Merge branch 'terrain-content' into infterrain-content

Conflicts:
	src/engine/terrain_chunk.cpp
	src/engine/terrain_chunk.h
engine/terrain: initial object placing support for the whole terrain
doc: added 3 more gameplay improvement ideas
engine/terrain: object placement improved for single chunk
engine/terrain: placing objects on any chunk implemented
readme: updated for better structure

also added project_structure doc file to help newcomers
configure: added note for package maintainers
doc: added no-rush setting idea