[✓][SUG][ED] Pop-out Piece Browser

[WillLem]

To complement the recently-implemented Pop-out Level Arranger, let's also have a Piece Browser which can be moved and resized (horizontally and vertically) as the user wishes.

At this point, I'm not entirely sure that it can be done quite as gracefully as the Level Arranger, since the Piece Browser has far more moving parts. But, it's definitely something for the to-do list.

[WillLem]

Implemented across several commits (starting with 3515e53, ending with 1a820da).

As with the Level Arranger window, the Piece Browser window can be moved to an external display if desired, and its size & location are preserved between sessions.

This feature will be available in the next update.