Make comparing timestamps of tracks more available.
Basic support of some GPX Metadata values common to GPX1.0 and GPX1.1

Includes the fields: name, description, author, time, and keywords.
ATM the time is a read-only value loaded from a GPX file (no GUI way to alter it).
If there is no value or it's a new TrackWaypoint layer then the value auto generated from the earliest track or waypoint
 or else from the current time.
Use TRW Metadata in generating default values for some OSM Trace upload fields.

Description and keywords are filled in from available values.
Fix opening files on command line in different directories.

Restrict the changing of the working directory when handling the Viking file type,
since that is the only type (as far as I know) that supports relative file paths
 (mostly for optional relative paths for Waypoint Images).
Restore the working directory to the previous value once the file has been used.