Runtime theme switching and support for theme extensions

[tomlin7]

Feature

Currently themes can't be changed at runtime, but will have to restart biscuit for theme change to take effect (infact to change the theme you have to restart anyways since the settings editor is not properly functional yet). The theme manager is in a don't-touch-it-works state, but we can't keep it like that, it does require a makeover.

As of now, I don't see a proper way to handle theme switching theme at runtime, unless we reconfigure every component of the editor from the theme manager (easy to access parts of the editor since the core app class / "base" holds access to every part of the editor including the layout manager)