Introduce tag alias

[youstanzr]

We can add alias tags in the setting (once #57 done) where it there would be alias and primary text field and add button that adds to a small table view. The primary is always whats shown in the filter picker but aliases of the primary is picked up when filtering for primary. Aliases should not appear in the filter picker view.

This is extremely helpful in cases that someone add a tag called 'oldies' for example and in another song, the user adds a tag called 'classics'. In this example, it would be easy to make 'classics' an alias of 'oldies' and everything would work nicely.