Implement simple search in playlist

[jeremypw]

Fixes #762

• Just adds a search entry to the queue header that appears whenever there are items in the queue.
• Entering a search term does a case insensitive search for the term anywhere in the titles in list and selects the first matching row (if any).
• If no match the currently selected item remains selected.
• The matching row does not play until the search entry is activated or the row is clicked on.
• The queue scrolls to show the matched item.
• <Ctrl>F focuses the search entry if showing

[zeebok]

This never seems to allow the first item in the list as a result to highlight, which is handy if I search in a list that is long and scrollable

[jeremypw]

@zeebok Sorry for the delay in getting back to this. I have now fixed the issue you noticed.