Default/mapgen: Add mapgen aliases for sandstone brick and sandstone brick stair, to enable sandstone dungeons

Organise and cleanup code

Register blob ores in mgv6 to replace addDirtGravelBlobs
Combine ores and blobs functions

Create register ores function. Select and order functions per mapgen
Use blob ore for clay. Update ore height_min/max to y_min/max
Tune flower amount

Signed-off-by: Craig Robbins <kde.psych@gmail.com>