fragezeichen2org: die on wrong usage of directive "#: prio"

"#: priority" is correct, and documented, and exclusively handled by
fragezeichen2org. As "#: prio" was used quite often, make it even a
fatal error, so it does not get unnoticed.
custom layer loader: new option for automatic scrollregion enlargement

The new option is checked by default. Should work in both lazy and
non-lazy modes.
new helper function: get_layer_by_abk
enlarge_scrollregion and lazy_plot

The special lazy plot case moved now to
enlarge_scrollregion_for_layer().
custom_draw/plot_additional_layer: -retobj not needed anymore
custom_draw: enlarge scrollregion before center
Strassen::GPX: set File member

With tests in t/strassen-gpx.t
strassen-gpsman.t: test dependent_files method
BBBikeSuggest: fixed the pre-5.6 compat code

Symbol::gensym was called, but never assigned to $ZIP. Fixed this just
for fun, untested due to lack of old perls.
PLZ->new_data_from_streets: handle multiple cityparts correct

Now there are _multiple_ lines for every citypart, instead of the
cityparts written out as a comma-separated list. This will probably
fix one BBBikeSuggest+SRTShortcuts bug.
new test file plzstreets.t

Testing the PLZ <-> Strassen functions, i.e.
PLZ->new_data_from_streets
data update