Improved 3D Scene Importer

* Added option for importers to show an Advanced settings dialog
* Created advanced settings dialog for Scene Importer
* Cleaned up importers (remove many old/unused options)
* Added the ability to customize every node, material, mesh and animation individually
* Saving to animations and meshes to files is now a manual process, making it more predictable
* Added the ability for materials to be replaced by external files (or to be made external, up to you).
* When doubleclicking an impoted scene in the filesystem dock, it automatically shows the import settings instead of asking to open it.

WARNING: Lightmap UV unwrap is not working, it needs to be re-made.

scene/resources/surface_tool.h

@@ -143,6 +143,8 @@ public:
 	void set_custom_format(int p_index, CustomFormat p_format);
 	CustomFormat get_custom_format(int p_index) const;
 
+	Mesh::PrimitiveType get_primitive() const;
+
 	void begin(Mesh::PrimitiveType p_primitive);
 
 	void set_color(Color p_color);
@@ -171,6 +173,7 @@ public:
 	Vector<int> generate_lod(float p_threshold, int p_target_index_count = 3);
 
 	void set_material(const Ref<Material> &p_material);
+	Ref<Material> get_material() const;
 
 	void clear();