Blender Git Loki

Fix: Unable to animate nodes modifier exposed properties

The RNA path used for animating the settings passed to the node tree
is incorrect. Currently it's just `settings.property_name`, but it's
the path from the ID, not the modifier, so it should be
`modifiers[modifier_name].settings.property_name`.

However, the "Settings" struct is separated in RNA and DNA, which means
that the callback to get the RNA path does not know about the modifier's
name in order to fill the above path, so some reference to the modifier
in the "Settings" struct would be necessary, which would create a
convoluted layout in the `ModifierData` struct.

Instead, this commit simply removes the "Settings" struct from RNA,
which isn't as elegant from the point of view of the Python API,
but otherwise it's a nice simplification. Note that we don't remove the
"Settings" struct from DNA, because it would break reading old files.

Differential Revision: https://developer.blender.org/D10175