added option to swap game elements in EM caves using V5/V6 EMC graphics

When having EMC level sets with EMC graphics that were originally
played with the EMC player program, but that have cave files in old
cave file format, setting "use_emc_tiles" to "true" causes certain
game elements to be swapped just like when loading cave files with new
EMC file format (version V5 or V6), like swapping dirt and grass and
some wall elements.

This is a level set option, not a graphics set option, so it is still
possible to override graphics for classic style EM level sets with
modern EMC graphics without game elements being changed (so dirt does
not change to grass in this case).