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.cpp

@@ -1059,6 +1059,10 @@ void SurfaceTool::set_material(const Ref<Material> &p_material) {
 	material = p_material;
 }
 
+Ref<Material> SurfaceTool::get_material() const {
+	return material;
+}
+
 void SurfaceTool::clear() {
 	begun = false;
 	primitive = Mesh::PRIMITIVE_LINES;
@@ -1088,6 +1092,10 @@ void SurfaceTool::set_custom_format(int p_index, CustomFormat p_format) {
 	ERR_FAIL_COND(begun);
 	last_custom_format[p_index] = p_format;
 }
+
+Mesh::PrimitiveType SurfaceTool::get_primitive() const {
+	return primitive;
+}
 SurfaceTool::CustomFormat SurfaceTool::get_custom_format(int p_index) const {
 	ERR_FAIL_INDEX_V(p_index, RS::ARRAY_CUSTOM_COUNT, CUSTOM_MAX);
 	return last_custom_format[p_index];
@@ -1174,6 +1182,7 @@ void SurfaceTool::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("generate_lod", "nd_threshold", "target_index_count"), &SurfaceTool::generate_lod, DEFVAL(3));
 
 	ClassDB::bind_method(D_METHOD("set_material", "material"), &SurfaceTool::set_material);
+	ClassDB::bind_method(D_METHOD("get_primitive"), &SurfaceTool::get_primitive);
 
 	ClassDB::bind_method(D_METHOD("clear"), &SurfaceTool::clear);