Make notes GPX valid and add useful official elements

Make notes GPX valid and add useful official elements

The "creator" attribute is required and "extensions" is the correct
name of this element.

Validated with SAXCount from the xerces-c package:

$ SAXCount -v=always -n -s -f notes.gpx

Original error messages:

    Error at file /home/markus/notes.gpx, line 2, char 171
      Message: no declaration found for element 'gpx'

    Error at file /home/markus/notes.gpx, line 2, char 171
      Message: attribute 'version' is not declared for element 'gpx'

After adding the "xmlns" attribute:

    Error at file /home/markus/notes.gpx, line 2, char 213
      Message: missing required attribute 'creator'

    Error at file /home/markus/notes.gpx, line 18, char 14
      Message: no declaration found for element 'extension'

    Error at file /home/markus/notes.gpx, line 26, char 7
      Message: element 'extension' is not allowed for content model '(ele?,time?,magvar?,geoidheight?,name?,cmt?,desc?,src?,link*,sym?,type?,fix?,sat?,hdop?,vdop?,pdop?,ageofdgpsdata?,dgpsid?,extensions?)'

The current errors now are caused by the missing XML schema for the
extensions.

Also add the time, name and link elements.