Seamless DLvl & dungeon branch transitions

[Mooses2k]

(going down stairs, ladder, rope, dolley etc, hot air balloon, rubble/skree slope, cave squeeze)

should keep current level, level above and level below in memory. When going down stairs, unload level above, and begin, in another thread, making the next level below.

• allow spiral stairs to go deeper than one level

References: https://github.com/godotengine/godot-proposals/issues/1197

EVEN MORE BASIC VERSION FOR DEMO:

• Put #103 basic var in place for branch_name = crypts, no connections, 5 DLvls
• Place an up stairs 3x3m room as the first room
• place a down stair 3x3m room far away,
• then run generation as normal.
• Player just walks onto the first stair, fade to black, load new random level keeping inventory
• Save levels so can go back up

[Mooses2k]

Have staircases, eh_jogos working on DLvls now. Choose to do fadeout fadein for demo, not continuous.