Add realtime tracking to GPS layer.

Add realtime tracking to GPS layer.

New features included:

- Show real time position from GPS.
- New layer menu item to start/stop realtime tracking.
- Using gpsd. Host and port can be set in the layer properties dialog.
- Enable/disable track recording (in peroperties dialog). Tracks are
  recorded in "GPS Download" folder.
- Jump to the first position on start (enable/disable in properties dialog).
- Moving maps to keep current tracking position at the center (enable/disable
  in properties dialog)

Signed-off-by: Quy Tonthat <qtonthat@gmail.com>