The old function didn't scale at all. As no replacements are going
to be merged anytime soon, start to make it meta-extensible.
This is the first step to be able to let the widget be extended
directly from rc.lua without adding yet more silly parameters.
It never scaled and has reached the point of no return a very long
time ago.
My first ever contribution to Awesome was to attempt to fix this,
but the solution was a bad hack. The radical module later solved
this by delegating the style, layout, theme, item layout and item
style to various "visitor" objects. While this is superior to this
commit, it was also a very large and complicated codebase. After
5 years, it is now obvious it will never be merged "whole".
8dedd19fcf