rework all settings pages to isolate each setting field as a stack of actions

With the current design of the settings pages in WordPress, plugins have a very hard time customizing the settings interface, as most panels have only a few specific places where custom code can be inserted via action.

To make this much easier to deal with, the built in settings pages should be reimplemented as a stack of render functions attached to individual actions per section.

These sections should also be attached by actions, so that any plugin can replace or augment a setting on a case by case basis, by simply adding new render functions to the same action, or by replacing the existing render function.