Completely remove SPOTMaps as this service is no longer available.

If it was available then it could work by simple maps.xml config,
 rather than needing any code.
Remove default usage of Terraserver maps as they are no longer available.

For now leave the code in place since there are extra functions,
 which may be of reference use in the future.
Remove no longer available map services.
Refix Merging tracks by time.

Accidentally broke it when trying to implement internal track ids.
(SHA: fc92c97715697980e8fde44b9dc8c02e29b1f7a0)

Somehow it previously did something, but it clearly wasn't actually
using *both* track times in deciding what to merge - d'oh!

It would also crash if one of the tracks has no trackpoints.
I think this flaw was in the original version.
This is now fixed as well.
Remove unnecessary subdomain in some OSM tileserver hostnames.
SF Features#91: Part 1. Display Clock Date/Time of the blob on the time graphs.

Helpful to work out the real time of where one was.
[QA] Refactoring track properties window.

Rename: MARGIN -> MARGIN_X, draw_grid -> draw_grid_y

Make resetting of the graph images a common function: clear_images().
Create and use a function to return the duration of a track.
SF Features#91: Part 2. Show time axis grids on time graphs.

Similar to the y axis labelling, try to find most sensible time chunks for the divisions.
Offset all drawing in the Y axis by MARGIN_Y to create room for the x axis labels.
Show distance axis grids on distance graphs.