Add operator that removes unused image maps in node trees

[Theverat]

Because we unfortunately have to use the fake_user on all imagemaps due to Blender's unreliable user counting on Python PointerProperties, Blender's built-in cleaning ceases to work.
Implement an operator that tries to find imagemaps with fake user enabled but no users.
Skip imagemaps that are linked in through libraries.

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[Theverat]

Node trees have the same problem, they also need cleanup.