Materials: External Module Support

The final PR for the external module feature that allows materials to
be stored in an external datastore, webservice, etc.

This includes the final material manager classes, and the UI support in
the form of commands and preference pages.

src/Mod/Material/App/ModelLibrary.h

@@ -46,7 +46,7 @@ class MaterialsExport ModelLibrary: public Library,
 
 public:
     ModelLibrary();
-    ModelLibrary(const Library& other);
+    ModelLibrary(const Library& library);
     ModelLibrary(const QString& libraryName,
                  const QString& dir,
                  const QString& icon,