Blender Git Loki

Add remaning 'init' callbacks for needed ID types.

Note all this remains a bit theoretical, some cases (ID types) are a bit delicate to handle here.7
Time and tests will say if it's OK as is.

Add some checks in BKE_xxx_init() that data is really NULLified.

Needed, since trying to (re)init data already set could lead to many issues,
and handling this is not in the scope of this function.

Note that definition of BLI_memcmp_null stuff is rather bad currently, no good idea where to place this. :|

Very first step to handle missing libs/datablocks when reading a file.

Idea is, instead of ignoring completely missing linked datablocks, to
create void placeholders for them.

That way, you can work on your file, save it, and find again your missing data once
lib becomes available again.

Plans are also to be able to locate missing libs and reload them at runtime.

To support all that, we must be able to make Blender survive those missing data
(i.e. empty datablocks) all over the place. This commit contains some early work
in that direction, but this will need much much more work!

Icons: Add new 'library_data_broken' one.