Pull out nifformat_to_mathutils_matrix, improve comment

Anim system improvements

only works with armature animation at the moment (force import skeleton only)
imports different controller sequences as separate actions
change priority of interpolator vs controller

Release/v0.0.11

Removed 'default' argument from collision_layer EnumProperty declaration. It wasn't doing anything and errored the plugin in Blender 2.8.

Made shader property flags interace dynamic, to avoid spelling mistakes and mismatches on format update.

Fixed issue where export would error if imported with an empty name on a root node due to mismatch between object and skeleton root name generation.

Changed bslightingshaderproperty slot 6 export to agree with import. Changed texture export to handle relative file paths, and ignore missing texture folder if no file found (i.e. path is likely directly from import).

Fixed issue where scales of different sign, but same size were not detected. Fixed issue where finding texture paths would take a long time (~20s).

Fixed name error of Facegen Detail Map property, causing it not to be exported. Added condition to not export normals when using a face tint (as done in face meshes).