GreasePencil v3 requires some additional processing on its data before
writing to blendfile, and after reading from it.
Extract this from generic `read/write` code, and put it into dedicated
callbacks in Cpp classes that need it (layer groups and leaves
currently).
NOTE: Work done while exploring possibility to use 'memfile' read/write
code for GP editmode undo (!117072), but ended up taking a different
approach for it, so this PR is purely a 'no expected changes in behavior'
refactor at this point.
Bastien Montagne