Fix T77330, T81704: Spline IK doesn't preserve bone scale correctly

Previously, the bone position outside of "fit to curve length" mode was
incorrect.

It assumed that the curve was completely straight with no bends or
turns. This would lead to bones being scaled down as their final
position would be servery underestimated in some cases.

The solution is to do a sphere -> curve intersection test to see where
to put the bones while still preserving their length. As we are using
the tessellated curve data this essentially boils down to us doing a
sphere -> line intersection check.

Reviewed By: Sybren

Differential Revision: http://developer.blender.org/D10849

Add ability to get a selection list of bones

This adds the ability to get a selection list for both edit mode bones
and pose mode bones.

To do this the selection menu list logic had to be reworked a bit.
Before it only stored the names of objects. This might work will of
objects, however as stated in the code, it might fail for linked objects
(so multiple object can have the same name in some corner cases).

For bones it is a very common occurance where you can have multiple
armature that has the same bone names. So now it also stores the object
and bone pointers for this case.

Reviewed By: Sybren

Differential Revision: http://developer.blender.org/D10570

Fix T85796

Fix T81707: Spline IK Joints "Floating" above curve

The issue was that where_on_path uses a resampled curve to get the data
from the curve. This leads to disconnects between the curve the user
sees and the evaluated location data.

To fix this we simply use the actual curve data the user can see.

The older code needed a cleanup either way as there were hacks in other
parts of the code trying to work around some brokenness. This is now
fixed and we no longer need to clamp the evaluation range to 0-1 or make
helper functions to make it do what we actually want.

Reviewed By: Campbell, Sybren

Differential Revision: http://developer.blender.org/D10898

Fix T84520: Make the different weight paint code paths exclusive to each other

Before this change, you could have the new sculpt symmetry code and the
older weight paint symmetry code active at the same time. This would
lead to users easily trashing their weigh paint data if they were not
careful when switching between modes.

Now the specific weight paint symmetry code is an exclusive toggle so
the user can't accidentally mirror strokes and vertex groups at the same
time. This also paves the way of supporting Y and Z symmetry in the
future for weight groups mirroring if we decide to add it in the future.

Reviewed By: Sybren

Differential Revision: http://developer.blender.org/D10426

Fix "unused variable" warning when compiling without nanovdb

Fix T86884: Don't remove the "line art object add" enum item

We would not add the "object line art" enum to the operator enum list,
if there were no active object in the scene.

This would make it impossible to call this operator from python code as
the enum would we hidden when we were not in a viewport context.

Always make the operator available, having no active object is not a
strict requirement for the operator to work, so expose it always.

Merge branch 'master' into temp-lineart-contained

Cleanup: Collection check return type in button_context

Merge branch 'master' into temp-lineart-contained

Merge branch 'master' into temp-lineart-contained