The elvdocs still use the old format (#elvdoc:fn or #elvdoc:var) for now, but
will be changed to "fn" and "var" forms soon.
Also remove the accidentally committed cmd/mvelvdoc. It has been used to perform
the conversion automatically but is not supposed to be committed.
* Move all widget types to a new pkg/cli/tk.
* Move Mode{Line Prompt} to a new pkg/cli/mode.
* Move all packages in pkg/cli/addons to pkg/cli/mode.
* Remove WriteListing.
* Remove StdTTY.
* Rename GetCodeBuffer to CodeBuffer.