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

Fixed spelling error in slsf1 'enviroment mapping' shader flag which caused incorrect import of the environment mapping flag (probably export too, but couldn't test that)

Removed unnecessary register functions.

Split out the various classes from the single UI file into multiple
modules to increase code cohesion. Added check for Mesh type for
partition panel rendering

niftools.object menu

settings. Unified import and export error classes to nif_utils.NifError
for sanity sake. 

renamed a few variables 

export

collision match, constraints are near perfect match. Still a slight
deviation on twist_b and pivot_b

those that matched existing blender conventions to those value items.

Added new custom flags and changed def names to better match current
use.

have determined their purpose. not sure if we need to worry with coding
them in.
Nif using the BSShader and dismember body parts import and export with a
few mismatches
Added custom to handle the alpha flag value

expected. Nifs with BSShaderPPLightingShader import and export, some
minor mismatches remain.

bsshader property set on export. More updates to the properties so that
set and get is easier.

properly. re applied select to collision import blocks, due to oops.

masking. nif root armature and bone now hold unique flags matching
original values.

armature, bsxflag for root node, upb string importing and exporting
intact.

mesh, and bone objects. RE added fix for radius calculation on nipacked
nif.

physical nifs which were left over from testing.