Clear up dispatch process

[jeremyharris]

It's very unclear what's happening in the dispatch process at this point. There are many actions and filters being set up during the theme class instantiation, some of which are called multiple times, some of which are triggered out of order (i.e., parent class fires before/after child class, specifically after_setup_theme).

We should reduce the amount of initial actions set up and move them into logical methods that can be overridden in child classes.

[scj1091]

Todo: Write a spec for the theme instantiation / dispatch process, to determine what elements Jeremy was referring to.