Change wording of "Add to Queue" button in Playlists tab

[clounie]

Changing it from Add to Queue --> Add Playlist to Queue would make it more clear what it's supposed to do.

It was exactly what I wanted to do, but I hesitated at first because the wording was ambiguous to me.

[nir-zilberman]

@JasonZigelbaum, I noticed that the wording now says Add All to Queue, so I guess this issue can be closed