qtui: added missing break; in switch statement

As Pitxyoki pointed out, "It might work, but it seems wrong to fall through like that.".
qtui: renamed deleteCurrentIndex function to deleteCurrentSelection

It makes more sense this way since playCurrentIndex refers to the playlist number the player is playing/going to play while deleteCurrentIndex deletes the playlist entry highlighted.
Merge pull request #37 from atomnuker/master

qtui: minor fixes to playlist