PATCH: Preferences/Options dialog

Please find the patch adding support for the preferences/options dialog.

Initial patch was shared by Khushboo with me, but - I made some design changes.

With this patch:

* Each preferences can be saved/retrieved per module basis.

* An object is created which represent that module in the preferences by default.

* Module needs to override the register_preferences(...) method to register certain preference.

* You can access other modules preference using static methods of the preferences.

* A collection module (PGChildModule) will register show_node_<NODE_TYPE>, and also refers the 'show_system_objects' in it, which can be accessed using self.pref_show_system_objects, or can use class property 'show_system_obejcts', also there is one more property 'show_node', which uses the 'pref_show_node' object of that module.