Fix main scene not being saved when set via Play button

Fixes #1134 for real. Also tweaked the engine.cfg template
and the preference order when browsing for the main scene.

core/globals.cpp

@@ -1429,7 +1429,7 @@ Globals::Globals() {
 
 	set("application/name","" );
 	set("application/main_scene","");
-	custom_prop_info["application/main_scene"]=PropertyInfo(Variant::STRING,"application/main_scene",PROPERTY_HINT_FILE,"scn,res,xscn,xml,tscn");
+	custom_prop_info["application/main_scene"]=PropertyInfo(Variant::STRING,"application/main_scene",PROPERTY_HINT_FILE,"tscn,scn,xscn,xml,res");
 	set("application/disable_stdout",false);
 	set("application/use_shared_user_dir",true);
 

tools/editor/editor_node.cpp

@@ -1175,6 +1175,8 @@ void EditorNode::_dialog_action(String p_file) {
 		case SETTINGS_PICK_MAIN_SCENE: {
 
 			Globals::get_singleton()->set("application/main_scene",p_file);
+			Globals::get_singleton()->set_persisting("application/main_scene",true);
+			Globals::get_singleton()->save();
 			//would be nice to show the project manager opened with the hilighted field..
 		} break;
 		case FILE_SAVE_OPTIMIZED: {

tools/editor/project_manager.cpp

@@ -203,14 +203,15 @@ private:
 				} else {
 
 					f->store_line("; Engine configuration file.");
-					f->store_line("; It's best to edit using the editor UI, not directly,");
-					f->store_line("; becausethe parameters that go here are not obvious.");
+					f->store_line("; It's best edited using the editor UI and not directly,");
+					f->store_line("; since the parameters that go here are not all obvious.");
 					f->store_line("; ");
 					f->store_line("; Format: ");
 					f->store_line(";   [section] ; section goes between []");
 					f->store_line(";   param=value ; assign values to parameters");
 					f->store_line("\n");
 					f->store_line("[application]");
+					f->store_line("\n");
 					f->store_line("name=\""+project_name->get_text()+"\"");
 					f->store_line("icon=\"res://icon.png\"");