Add view for incremental filtering

[muesli4]

Searching the playlist is a little inconvenient. Instead enable filtering by different criteria. There are different ways one could implement this from a UI perspective:

• A series of dialogs where you progress or skip through a predefined set of dialogs (artist, title, genre, etc.) or a tree style menu where arbitrary criteria can be added to the query.
• Results may be displayed after the query or incrementally.
• MPD is well-suited for such queries. The widget should obviously be decoupled from the MPD specific parts via callbacks or other means.

One advantage I see in incremental searching is that it would limit the choices taken in the next step. So overall, less input is required from the user. Feedback may be provided directly.