rooms definitions: add "cursor move" command to introduce in-between positioning along --/-/+/++

[meszaros-lajos-gyorgy]

Currently the rooms can only be aligned either to the center of the other room or to its edges:

It's nicer to have a bit of space around the entrance to have wall support on all sides:

This can only be achieved - as the rooms must align to the edges - by adding rooms with one or two dimensions set to 0 just for offsetting the internal cursor. Example from LaLee's minigame:

# room C - pantry
cursor restore living-room
room add 0 0 50 spacer z- x++ y-
room add 50 $entranceHeight 200 door z++ x++ y-
room add 600 $roomHeight 350 room-with-tiles z- x++ y-
with light dim
room add 600 $roomHeight 50 room-with-tiles z-- y-

This hack could be turned into a proper feature and could be done cleanly with manipulating the cursor directly from code:

cursor move 0 0 50

Internally it could be implemented as adding a room as big as the previous one, but with no walls/ceiling/floor and the coordinates adjusted with the 2 values. This way cursor save would also work here.