Add support for nodes tagged with seamark:light:character= Fl and seamark:light:period tag

[woll]

I guess the code ignores nodes with a seamark:light:period tag and no seamark:light:sequence tag.
There are quite a lot of lighthouses (with a simple flash), that are only tagged with seamark:light:period without seamark:light:sequence, so it would be good to support those.
e.g. before I edited it (to add seamark:light:sequence) Douglas Head lighthouse (which has a simple flash every 10 seconds) had the following tags, but it didn't appear on the map:
seamark:light:character= Fl
seamark:light:colour=white
seamark:light:height=32
seamark:light:period=10
seamark:light:range=24
seamark:light:reference=A 4770
seamark:type=light_major
source=US NGA Pub. 114. 2011-05-26

There are different values for seamark:light:character that might complicate full support, but a tag of seamark:light:character= Fl is a simple momentary flash (seemingly tagged as a standard 0.5 seconds flash for lights that also have a seamark:light:sequence tag) with the given period.