This fixes a violation of module layers (circular dependency)
introduced in e6cd13d7e13775de2a687e55e59819cb7f19e7e9 where doc-lib
started to depend on app-lib (which cannot happen, as app-lib depends
on doc-lib).
This DocFormat/SerialFormat was used only from app-lib previously, but
when properties were introduced in user data, the serialization format
version was needed to read user properties too. So now it makes sense
to move this type/its values to the doc-lib.
A couple of extra issues were found in this refactor:
1) The recursive call inside read_layer() didn't receive this "serial"
argument
2) read_grid() doesn't need the setId parameter
