Reduce duplication by using magic get/set methods in main bbPress class

[johnjamesjacoby]

The main bbPress class sets some default variables, then overrides most of them in the setup_globals() method. This is wasteful, and that data can all just be set inside of setup_globals.

While we're moving things around, we may as well move everything into using magic getters and setters. The only exception to this is bbpress()->current_user, which is passed by reference.

[johnjamesjacoby]

(In [3951]) Code Improvement:

• Move preset bbPress class variables into bbPress::setup_globals() action.
• Introduce magic get/set methods to handle main bbPress class variables.
• Keeps by-reference bbPress->current_user outside of bbPress->data.
• Reduces code duplication in bbPress class.
• Fixes #1847.