Implement initial TCX TrackPoint data

Still a number of @todo's to go, including some required elements for
the multisport mode.
Complete basic TCX implementation

TCX conversion should now be complete, in theory. Just need a bunch of
cleanup, error handling, and additional (optional) features.
Bounds check all unsafe QList method calls

Also add some const's here and there.
Remove unnecessary operator[] calls

These violate constness unnecessarily, and in some cases result in
unneeded deep copies.
Bounds check remaining QList::at calls
Remove excess white space
Add GPX metadata/author/link element
Link to XML schemata
Add a *really* basic UI
Implement basic headless / UI-less convert-all mode

Nice! :D