Now that the plugin works and I'm done with the main development, move
everything from `plugin/` to `autoload/`.

Makes the "Tab page X" lines appear in the same colour as they usually
do when using the `:tabs` command.

I previously extracted this check to `s:IsTabPageLine()` in order to use
it in `s:FilterTabPageElements()`. Eliminate it from `s:MatchString()`
now and reuse the function.

This was never really useful, just a holdover from very early
development (copied from LsGrep and modified a bit).

Remove development comments. And a debug `echo`. Add more descriptive
comments for the different steps.

This function now works the way I want it to, removing the "Tab page X"
headers with no files under them from the `:tabs` list.

We do this by building a list of indexes of those "Tab page X" elements
and then removing them from the original list. The removal happens in
reverse so we don't mess up the indexes by removing from left to right.

Add a function that will filter out "Tab page 10" elements that don't
have any files under them.

Incomplete for now but I'm committing what I have here because it looks
like I'm going to have to change things around just a bit.

At the moment, the function will take the list of matched `:tabs` lines,
make a list of the indexes of the "Tab page X" elements, and finally
remove the empty "Tab page X" elements from that list, leaving only the
indexes of those with files under them.

Instead what I think I need is a list of the "Tab page X" indexes that
don't have files so I can remove these from the list.