Database: introduce Marker.TrackPointId column

[dennisguse]

For (unknown) historic reasons, the Marker's table links to the TrackPoint table by storing

• TrackId
• Latitude
• Longitude
  In case there are duplicate TrackPoints (e.g., visiting the same place twice during one track recording), the first TrackPoint is chosen.

I guess, back then this kind of convenient (it is also in line with the exports in KML/GPX).
This is however, not that great as it kind of tricky to maintain.

The goal is to simplify this, by replacing Marker's.latitude/longitude by introducing Markers.TrackPointId.

Affected Modules:

• Import/Export (KML/GPX)
• ChartView
• MarkerList
• CustomContentProvider / ContentProviderUtils

PS/ as TrackPoint.Id is unique within OpenTracks, Marker.TrackId may be dropped as well.
However, this column may have some performance benefits.

[dennisguse]

#1831 (comment)