[3.3.0-dev] Possible to have more than one Playlists with the same name

Steps to reproduce

1. Create a Playlist called "test" with some medias
2. Choose other medias and create a Playlist by typing in "test" to the name.
3. In Playlist screen, we have two Playlists with same name

Expected behavior

Only one Playlist called "test" should have been created with union of the two set of medias.

Probable cause

In the SavePlaylistDialog, the Playlist is obtained from the selected ID. If not selected anything, then it creates a new Playlist with the typed name. It would also create issue when person has both selected an existing Playlist and typed a new name, since it only looks for selected ID.