Now callbacks for animdata, nla strip and fcurve are in their own proper
BKE files (mimicking `foreach_id` callback of `IDTypeInfo`).
This commit also fixes some missing handling of ID pointers (text ID and
IDProperties of script fcurve modifier...).
Surrounding includes with an 'extern "C"' block is not necessary anymore.
Also that made it harder to add any C++ code to some headers, or include headers
that have "optional" C++ code like `MEM_guardedalloc.h`.
I tested compilation on linux and windows (and got help from @LazyDodo).
If this still breaks compilation due to some linker error, the header containing
the symbol in question is probably missing an 'extern "C"' block.
Differential Revision: https://developer.blender.org/D7653
The `BKE_animsys.h` and `anim_sys.c` files already had a an "AnimData
API" section. The code in that section has now been split off, and
placed into `BKE_anim_data.h` and `anim_data.c`.
All files that used to include `BKE_animsys.h` have been adjusted to
only include the animation headers they need (sometimes none).
No functional changes.
