Added material and material collection item classes.
Give MaterialCollectionItem base functionality

This is a temporary commit. Probably won't compile.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Different editors for material types

Currently this is available only at creation. The opening of a created
material is still not working.
Added submenu for Materials.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Added slot_edit, to be implemented

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>