Fix #202

As the theme's documentation is generated from `doc/metropolistheme.dtx` and not by running latex on the component sub-packages, this code is never actually executed. (Good thing, too, since it wouldn't compile; it is not possible to call `beamerthememetropolis` in a document with the `ltxdoc` class)

Recent changes reduced it way more than it used to be, leading to a cramped
feeling especially with small caps titling.

This reverts the portion of the commit c48cc90a that applies to the outer theme, in order to prevent merge conflicts for #183

Added possibility to add a custom footer. Usage is shown in the demo slide deck.

Fix #126

Although (e.g.) `titleformat title` is a valid PGF key and works in the `\metroset`, it does not work as an optional argument to `\usetheme`, where spaces are ignored.

Use \metropolis@foo instead of \@metropolis@foo, following the example of`beamer`'s source code

Use \themename in documentation instead.

`vphantom` implementation proposed by @rchurchley in #115

I’ve put a `\@metropolis@frametitlestrut` at the beginning and the end
because of what I can see will be an error for multiline titles.
Perhaps what @benjamin-weiss was alluding too?

If we were to put the strut at the end of a multiline title, then if
the first line was nothing but lower case characters (for some reason…)
in a `frametitleformat=regular` situation then the strut would make
sure that the minimum height would still be that of a capital letter.
But, since this is a multiline title that only happens for the last
line.

If we thing of the opposite situation; the strut is at the beginning
but our final line is nothing but lowercase. Then the strut again makes
sure that the minimum height is that of a capital letter. But, this is
only applied to the first line now. To rectify this I’ve put a strut at
both the beginning and the end. Now, this won’t help If the presenter
has a multiline title that is three or more lines long (god forbid) but
in that case the standard leading and `\baselinskip` should take care
of it. Also, the central line(s) shouldn’t effect the padding around
the title anyway.