Delete output pdf on build failures

See merge request !2

This is intended as a replacement for the call of the clean target in the
i4neo-demo Makefile. The call of clean is problematic as this triggers
both the clean target in i4neo-demo and the cache in the i4neo theme.

The dependency on the cache dir only serves to ensure that the cache
directory exists. Using a normal dependency, however, causes make to
rerun every dependent task once any file in the directory has changed.
This usually causes lots of unnecessary rebuilds. Fix this by specifying
the cache directory as order-only dependency which avoids rebuilds due
to changed dependencies.

Up to now, the alignment was done using \hfills, which will cause the
shorttitle to be centered between author and pagenumber, but not
centered with regards to the current page.
This commit introduces a tikz overlay instead. Please note: for the
standout-style, the bar has to be drawn twice: This is the only way to
draw the background and text on the main layer, without resorting to
pgfonlayer, which might have the unfortunate result of mixing with
included tikz graphics.

This allows TeX-Editors to sync the TeX and PDF views.

The previous spacing used by the plain style was way too large.

Restrict standout font and alignment to inner content

Restore theme within frame, making standout slides work with ignorenonframetext

So that the speaker's notes aren't affected by the typesetting of the slide.

Restore theme within frame, making standout slides work with ignorenonframetext

Update URL to Paul Tol technical note

The BSD /bin/echo doesn't know the option '-e'

This ensures that the whole text of a footnote has the same indentation
and not just the line following the footnote mark

The upper part of the slide is white text on dark background.
For the title graphic part it's the other way round, so switch
the text color back to black.

Updated Fira font paths

Fixed docker build command in Makefile.