Add GPS tracking plugin

The plugin can be build by passing the --enable-gps flag to the
configure script. The defaults is disabled for now.

The GPS Plugin:
  - Displays the GPS status and location in the config area
  - Draws the current location in the viewer using a marker
  - Optionally displays a tracks of previous locations
  - Optionally follows the current location
  - Optionally logs the location to a text file

The GPS plugin uses version 5 of the GPSd protocol, so it is only
supported under Linux at this time.