front: Remove md5 field from RomDetail (avoid all ambiguity).

Remove all chance that we are ever using an alias MD5.  We are always
passing around the true, calculated MD5 for the ROM.
front: Add convenience constructor to RomHeader.
front: No longer need to pass CRC to PlayMenuActivity.
front: Only store essential information in GalleryItem, no db lookup.

Speeds up loading and minimizes coupling.
(wip) rename game directory