The hook that defines the "lastslide" label is interfered with for
some reason on an updated TeX Live 2020. We thus avoid referencing
the label and instead show only the current slide number on each
slide. We leave instructions in the preamble for how to get back the
previous behavior if desired.
Re-structure and rename files in a transparent way. Most template/example
names now correspond to the (verbose GUI) name of their layouts.
Note that this, most prominently, also changes [LANG/]splash.lyx to
something less insiderish, namely "Welcome.lyx".
