Timecode fixes

[Bart Cerneels]

Hey Ramblurr,

You probably saw in the irc backlog we were discussing the Timecode
(Track position saving using the Bookmark system) features today.

Markey wanted to disable it for the beta because it creates to many
Timecodes for one Track. We've agreed on the following list that needs
to be fixed in order to be included:

- Only one auto-timecode per track:
   The use-case is starting a long track (podcast episode, audiobook,
prog. rock track, ...) right where you left it after a stop or
(accidental) track change. So we don't need multiple auto-timecodes,
only the most recent stop position. Delete the previous one before
creating a new one.
- Delete the auto-timecode when a Track completes:
   No need to keep it in the database when the user has no more need for it.
- Automatically jump to auto-timecode position:
   Use Scenario: I stopped listening in the middle of a podcast
episode and want to resume. Easy to do when there is only one.
- Rename the user strings to "Position Marker":
   This term makes the most sense for the common audience. Timecode is
to technical (though a nice class-name). Bookmark is to generic and
makes people think of web browsers.
- Delete timecodes right from the progress-bar area:
Using some inline delete button in the hover-over widget (Artists:
please make some mockups.). Please remember that hover over does not
exists on touchscreen, so it should not be the only way to delete. I
think the bookmark manager is also not a good option because to
complex and undiscoverable.

Bart (channeling the irc-channel)