Refactor: passed around aggregate menu values
Refactor: passed around map layer menu values
Only the TrackWaypoint layer should handle it's internal callback type.
Remove never used GPS layer drag drop request function.

Seemingly never used and it's not clear what it was meant to achieve.
Even if it was invoked, it would have aborted the program anyway:
 as it was the only user of vik_gps_layer_delete()
 which would have failed on the number of disconnect signals never equalling 1.

So remove vik_gps_layer_delete() too.
Improve update signal handler usage in GPS Layer.

Remove unnecessary addition of update signal handler on copy,
 which is why previously the number on disconnect was greater than expected.
Tidy update signal handler in VikAggregate Layer.
Refactor: TrackWaypoint layer menu callback functions.
Refactor: TrackWaypoint sublayer menu callback functions.
Refactor: Create and use functions to access track start or end trackpoints.
[QA] Remove compiler warning when building for Windows
[QA] Remove unnecessary g_critical()
Merge branch 'Refactor'