Cleanup: clarification of 'name' in BKE_idtype functions

The 'name' parameter of `BKE_idtype_idcode_from_name()`, and the `str`
parameter of `idtype_get_info_from_name()`, are expected to be the
'user visible name' of an `IDTypeInfo` struct. This is made clearer in
the code by renaming those parameters to `idtype_name` and mentioning
it in the documentation of the `BKE_idtype_idcode_from_name()`
function.

Differential Revision: https://developer.blender.org/D7317

Cleanup: Animation, move AnimData API to `anim_data.c`/`BKE_anim_data.h`

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.

Cleanup: add missing #includes to some headers

It should be possible to `#include` any header without having to worry
about its dependencies.

I didn't go and check all include files for this, just the ones that caused
me errors while I was refactoring the `anim_sys.c` file.

No functional changes.

Cleanup: split `BKE_anim.h` and `anim.c` into smaller pieces

The files are now split up into the following sections:
- `BKE_anim_path.h` and `anim_path.c` for path/curve functions.
- `BKE_anim_visualization.h` and `anim_visualizationanim_path.c` for
animation visualization (mostly motion paths).
- `BKE_duplilist.h` for DupliList function declarations. These were
already implemented in `object_dupli.c`, so they were rather out of
place being declared in `BKE_anim.h` in the first place.

No functional changes.

Cleanup: Font, added initialisation for two variables

My compiler (GCC 7.5.0) was warning about these variables potentially not
being initialised. Since the function is highly complex, instead of
analysing it I just trust my compiler and added initial values.

This should be no functional change.

Fix T74983: Material preview icons don't refresh

The root cause of the issue reported in T74983 is that an `IDNode` would
not be marked as user-modified. This marking happened while looping over
outgoing relations of one of its operation nodes. Since rBff60dd8b18ed
unused relations are removed, and as a result the `IDNode` would not be
marked.

The solution was to move the responsible code outside the loop; this is
probably a good idea anyway, as the code did not actually use the
looped-over relations at all, and was thus repeated unnecessarily.

Cleanup: reduce code indentation in autokeyframe_pose()

This makes the code a bit simpler to follow, by replacing
`if (x) { all the code here }` with `if (!x) { continue; }` and un-indenting
the remaining code, and by returning early.

No functional changes.

Fix T75142: No autokeying with pose mode X-Mirror

This was caused by the removal of some `BONE_TRANSFORM_MIRROR` flag
handling in rBde530a95dc7b482dc22c933b9b8b2a98c79b5663. I simply
restored those lines that caused this issue.

Alembic Export: Removed TODO

Fix crash when exporting

The OAlembic object was copied, and my assumption that this would work fine
was wrong.