A completion is now a table with attributes :text, :widget, :value
A completer returns an array of completions
This means we can now render URL completions with the page title
as well as the URL
lots of dependent changes here, unfortunately
- get rid of Buffer.current (it's per-frame now)
- in define-command, functions providing default parameter values
now accept frame as parameter
- in keymaps, parameter values are functions that accept frame as
parameter
instead of using functions directly. This is so that the
appropriate frame-relevant commander can be used to execure
the command, but also makes the keymaps a little less opaque
