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.