Rename the "Reload from Musicbrainz" option in the context menu and opti...

...mize MB dialog, https://bugs.launchpad.net/mixxx/+bug/1184157

* Add separator in the context menu to group metadata related menu points
* Rename available options to "Reload Metadata from File" and "Get Metadata from MusicBrainz"
   Since all options in this menu are somehow related to a track, it was reasonable to drop the extra  "track" from 
   both menu points.
* In the MusicBrainz fetcher dialog:
   - Add descriptive Title to the dialog window
   - Add some translator comments
   - Change Song -> Track
   - Change button order to be like in the Tracks Properties dialog
   - Use Ampersands (&) for button text to quickly access that particular GUI element.
   - Fix tab order.
   - Removes styling for some text in error messages. We already have different styles in the preferences dialogs (which is bad and needs clean up), go with the system default instead.
* In the Tracks Properties dialog, metadata buttons are now in the same order like in the context menu. Also fix tab order.