Created a tag editor for local songs
Close the tag editor when we've finished editing
Reflect tag changes in the UI immedtately
Move the save button to the left
Show an overlay with a progress indicator when saving song metadata
Handle any error when saving the tags
Refactored the metadata selection code
Display a warning when the user is trying to save the metadata of multiple songs
Merge pull request #123 from flagbug/tag-editor

Created a tag editor for local songs