Refactor code for selecting the tabs to which a tab command with a count
prefic should apply.

For example, search the current site:

    cd: javascript:location='http://www.google.com/search?num=100&q=site:'+escape(location.hostname)+'+%s'

Apparently an example like this has been on the Wiki for four years, but
it has not been supported.

However, the change is so trivial that it's worth doing anyway.

Fixes #2956.

According to:

  - https://bugzilla.mozilla.org/show_bug.cgi?id=1253129

and #2896, this:

1. causes Firefox to crash, and
2. isn't actually necessary.

So just remove it.

H/t @ris58h in #2896.

Move the `reload` command to the background page and make it accept a
count prefix.

Fixes #674.

Althought the demand for this is low and the use cases limited, it is a
very vim-ish extension to the existing command.

tabConfig doesn't exist in this scope.

On the options page, treat all user-defined mapping commands as lower
case.

Why?

- `unmapAll` is not very vim-ish, and is inconsistent with `mapkey`.
- If we were to introduce `vmap` for visual mode, then we would get
`vUnmapAll`, which is hideous capitalisation.

Possible down side?

- I guess somebody might have disabled a `map` by capitalising it.

Specifically, fix createTab with multiple URLs specified.

Fixes #2868.

This puts link hints frame behaviour back to matching
fb00eaa6bd4ee8889d10a9ef9d976fefd3be7879

In particular, we go back to not sending link hints messages to frames
that are too small (according to DomUtils.windowIsTooSmall).

This is a more complete fix for issue #2125.