Update Project Settings page

- Add screenshots
- Document reading and writing settings
- Document manually editing project.godot
- Note that most settings are changed in different
classes at runtime

tutorials/plugins/editor/installing_plugins.rst

@@ -53,6 +53,8 @@ plugin's ``addons/`` folder into your project folder to merge the new folder
 contents with the existing one. Your file manager may ask you whether to write
 into the folder; answer **Yes**. No files will be overwritten in the process.
 
+.. _doc_installing_plugins_enabling_a_plugin:
+
 Enabling a plugin
 ~~~~~~~~~~~~~~~~~