USD: replace check_is_transform_animated() with BKE_object_moves_in_time()

Merge remote-tracking branch 'origin/master' into temp-sybren-usd-patch

USD: typedef for std::map<short, pxr::VtIntArray>

USD: use BKE_object_visibility()

USD: Clarified some comments, distinguish Hydra from USDViewer

USD: removed dead code

USD: added TODO about checking depsgraph for time dependency

USD: use `BKE_animdata_id_is_animated()`

`BKE_animdata_id_is_animated()` is now used to determine whether object data
has any animation. This also includes counting the fcurves+drivers; previously
the presense of a non-NULL AnimData pointer was enough to consider the
object data animated.

Merge remote-tracking branch 'origin/master' into temp-sybren-usd-patch

USD: clarification of USDAbstractWriter::is_supported()

USD: iterate() ? iterate_and_write()

USD: 1024 ? FILE_MAX

USD: avoid term 'original' in non-CoW context

Instead of 'original object' the code now mentions 'source' or
'duplisource'.

USD: renamed `prune_the_weak` ? `remove_weak_subtrees` + arguments

USD: rename `link` to `dupli_object` and made variable loop-local

I added an explicit `!= nullptr` to make the formatter put the three parts
of the `for`-statement on three different lines (instead of two), which
increases readability.

USD: made `bool weak_export` `const`

USD: removed terminal colour codes

USD: add unit test to test `HierarchyContext::operator<()`

USD: nicer structure for tests/gtests/usd/CMakeLists.txt

This structure makes it easier to add new test source files, and avoids
using a temporary variable.

USD: clarified ownership of objects returned by create_xxxx_writer()