wip: Get speed and heading from NMEA RMC sentences

Add functions in src/gclue-location.c and src/gclue-nmea-source.c to fetch speed from $GPRMC sentences.
Add relevant header files.

Fixes #48
4 jobs for dev in 3 minutes and 57 seconds (queued for 7 seconds)
latest
Status Job ID Name Coverage
  Test
passed #277984
ubuntu-16.04

00:01:40

passed #277985
ubuntu-17.10

00:01:54

passed #277986
ubuntu-18.04

00:03:00

passed #277987
ubuntu-18.04-no-backend

00:01:32