Move debug streams to log.cpp|h

Move GUI-related globals to clientlauncher

Move g_settings and g_settings_path to settings.cpp|h

Move g_menuclouds to clouds.cpp|h

Move g_profiler to profiler.cpp|h

Use/add stone_surface_max_y to speed-optimise/guide dungeon generation
MgV7: Don't let mountain terrain chop dungeons at mapchunk borders
Make mountain terrain update stone_surface_max_y for caves in mountains

Add 'absolute value' option to noise map functions
Extend persistence modulation to 3D noise
Extend 'eased' option to noise2d_perlin* functions
Some noise.cpp formatting fixups

Add support for notify-on-decoration
Clean up mapgen constructors
Clean up mapgen.cpp code style somewhat
Remove trailing whitespace from some files

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.