Split up ModApiMapgen::l_register_decoration()
Define and make use of CONTAINS() and ARRLEN() macros

This also cleans up settings a bit

Remove flagmask field from set_mapgen_params table
Add small bits of needed documentation

MapgenParams is no longer a polymorphic class, eliminating the need for messy and bug-prone reallocations.
Separation between the common and mapgen-specific parameters is now strongly defined.
Mapgen parameters objects are now properly encapsulated within the proper subsystems.

Decoration: Stop DecoSimple::resolveNodeNames from complaining about no node name if decolist is used

Fix warning message for spawnby nodes

Prevent type-punning warning caused by casting enum to int