Re-organize ID tags in a more logical way, and keep their values
strictly increasing, splitting the free available ones in-between the
main groups (to avoid having to edit all tags values when adding a new
one).
Note that shuffling around these ID tags values should not be an issue
anymore, all of these are strictly run-time, and fully cleared in write
code when writing into a .blend file.
This also lead to the second cleanup, which is removing some asserts on
ID tag values in readcode, these are useless since the tag is cleared on
write.
62548acb1a