Rewrite the epa key interface to use buttons instead of widgets

Rewrite the epa key interface to use buttons instead of widgets

* lisp/epa.el (epa-font-lock-keywords): Removed.
(epa-key-list-mode-map): Bind tab/backtab to button navigation.
(epa-key): Remove widget.
(epa--button-key-text): Return the propertized text instead of
return a widget text.
(epa-key-list-mode): Don't use font locking; everything is output
as it should be.
(epa--insert-keys): Rewrite to just output the data instead of
widgetising.
(epa--select-keys): Insert buttons instead of widgets.