Combine new playlist and smart playlist UI

Currently, the method for creating a playlist and smart playlist in Noise is a bit different. We need to combine them into one simple UI to make it consistent with other elementary apps that use the +- bar.

TODO:

Smart Playlist
* Use a popover instead of a separate window for making a new playlist
* Have the smart playlist icon in the entry to show that its a smart playlist
* Allow rules to be removed
* Make the playlist size limit a part of the rules
* When all the rules are removed, revert back to a normal playlist (show normal playlist icon in entry)

Playlist
* Instead having an entry to change the playlist name in the sidebar, use a popover with an entry to name the playlist
* The entry should have a playlist icon to show that its a normal playlist
* Have a button that turns this normal playlist into a smart playlist (this is the same as the "add rule..." button for smart playlists but with a different string