This is going to be a bit more comprehensive than anticipated. In order
to cleanly refactor the core and the engine, basically start from
scratch with the implementation.
Goals:
* Test coverage
* Maintain backwards compatibility with module interface as much as
possible (but still make modules easier to code)
* Simplicity
see #23
Add code that allows themes to be merged (i.e. if certain elements are
not present in a theme, another theme can be "overlaid" to add missing
elements).
Effectively, this is used to create the logical concept of an "icon
theme", which is loaded after the main theme. So, the main theme can
define colors, and the icon theme fills in any missing elements
(practically, all the icons in the form of prefixes and suffixes).
Icon sets are defined in a theme using the "icons" directive, which
should be an array.
see #17
