Allow manual entry of GPS coordinates

This implementation gives the user the ability to add GPS coordinates to
the dive location. It works as expected in multi-dive edit scenarios. It
has two major flaws:
- it only support WGS84 style coordinates - it should instead be
  intelligent enough to parse the common formats, at least using NSEW
  instead of sign and understanding minutes and seconds instead of decimal
  degrees
- but in reality, it really needs to support a map picker

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Parse an empty string as "no GPS coordinates"

That's much more intuitive to remove GPS data from a dive than having to
type in 0,0 as coordinates.

With this change we also skip leading whitespace for WGS84 coordinates.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
UTF8 aware parser for some more GPS formats

I'm sure there are better ways to do this, but this appears to grok most
rational formats I was able to find. NSEW or positive/negative numbers.
Decimal degrees (WGS84) or degrees and decimal minutes (that's what most
GPSs seem to provide). I'm sure there are still corner cases that confuse
it, but it seemed reasonably robust in testing.

I don't really love the ';' as separator but that solves the obvious
problem with locales that use a decimal comma.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>