Fixed saving of stagger index

Also, only save the hex side length for hexagonal maps, since it makes
no sense to appear for other map types.
Store the stagger direction, even though only 'rows' is supported now

This is mainly because I think 'columns' should be the default (it's
what Tiled Java did and as such may be supported by 3rd party libraries
this way). Hexagonal maps created by this version of Tiled are now saved
with 'staggerdirection' set to 'rows' explicitly.