Updates the material preferences including selecting a default material
Preferences adds some options for what gets displayed in the Materials editor.
The option to use the legacy editor is removed in favour of the new
editor.
A new preference page has been added that allows the user to select a
default material. This will then be assigned to any newly created object. In
support of this, a new widget PrefsMaterialTreeWidget has been added as an
extension of the MaterialTreeWidget to automatically save and restore the
selected material.
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