Basic support of some GPX Metadata values common to GPX1.0 and GPX1.1

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.