Refactor module classes

[herrvigg]

The module objects are very difficult to manipulate with simple arrays.

Reorganize the code for a much better structure:

• QTX_Admin_Module: provides the basic static data (id, name, plugins, ...) and the built-in setup.
• QTX_Admin_Module_Manager: manages the module state updates and notices
• QTX_Admin_Module_Settings: holds the data relevant for the settings, mostly for display
• QTX_Module_Loader: reads the state and loads modules (used by admin and front core)

Move files to modules folder to keep the related code together.

[herrvigg]

Refactoring helps to have a complete different view.