Factorized writing of ID block's data (so far, only IDProps) into own helper func.
This also fixes missing IDProp (aka custom data) saving from GreasePencil and Library
datablocks (and add comment about why we do not save WM IDProps).
Finaly, it ensures all ID-related data are written immediately after the ID itself
(was not the case for all data types previously, some were writting their own data
before IDProps). This is not a fix (.blend file format does not enforce any order
on sub-data of datablocks, they only have to be after datablock struct itself in file),
but makes things more consistent.
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