LibraryView: show message when no tracks in Library

- Store the search string in the main thread
- Make background of LibraryView 0.91% white
- Add a text field and a button that appears only when there are no
  tracks in the Library and no search is running
  - Button opens up the preferences through AppDelegate (same as menu
    item)
- Control visibility of library v. no-track-stuff with new function,
  updateLibraryVisibility
  - Called:
    - At the beginning
    - Every time the library is changed through the CoreData notifier
    - Every time a search is run or not run
  - Assumes that if the library is showing, then the no-track-stuff is
    not and vice-versa
- Related to issue #61