References to data-blocks in a material were stored in-memory and could
crash if the data-blocks referenced by the material no longer existed
when pasting.
Resolve by using a blend-file for material copy/paste, matching how the
clipboard works in the 3D view-port.
Currently there is no support for including indirectly linked
data-blocks when pasting the material. Instead, data-blocks are restored
by name, by inspecting the current file.
This also fixes a crash where the `SpaceNode::nodetree` could point to
freed memory when pasting a material.
Ref !108496.
Includes contributions by @mont29.
5177e2f20b