Swap ClipboardContext with ClipboardStore to copy to the system
clipboard.

Also add some documentation to the ClipboardStore::set_context function.

* Rename `last` to `original` because we only want to set it the first
  time we set the clipboard. Subsequent times we set the clipboard
  contents should not affect this saved value.
* Only set `original` on first run of the clipboard `set_contents`
  function.

The new type `ClipboardStore` wraps `ClipboardContext`, allowing us to
save the contents of the clipboard before resetting it. We will then be
able to reset the clipboard to this original value.

This commit includes some old code from when I was originally thinking
about this problem before I decided to make a separate module for it.
That code should be deleted.

One caveat to think about in the future is how to reset the clipboard to
the original value after a period of time. In other words, how to pass
the original contents value to a spawned child process.

Also, we need to make sure that we're only setting `last` once.
Subsequent calls should not change the `last` value. And now that I
think about that, "last" is probably the wrong name for this. It should
be something like "original".

`g`: moves the selection to the first option
`G`: moves the selection to the last option

This allows for faster selection if there are more than 2–3 options.

Using `g` still allows `gg` to work since doubling the command doesn't
change its meaning.

Values that contained ": " would not be parsed correctly for copying to
the clipboard. If, for example, a password contained ": ", those
characters and any following them wouldn't be copied. This change allows
such passwords to be copied correctly.