Adds implementation for material stacks.

Merged Eclipse Webmaster requested to merge github/fork/KasperGam/jay/reflectivity into jay/reflectivity

Created by: KasperGam

better implementation of composites. Updated UI to work with MaterialStack in the Materials Database Editor. Fixed bug 469296, bug 470405, bug 470407, bug 470409, bug 470411, bug 470412.

Signed-off-by: Kasper Gammeltoft

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