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

[esbrandt]

...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.

[rryan]

LGTM -- thanks!