Fix elided text in playlist headers and clean code and design somewhat
This MR allows album and artist names in playlist headers to become two-line strings before eliding. To accommodate this, it allows the header area to stretch vertically a small amount.
Doing this required a bit of code refactoring, as the header previously had a hardcoded height. Design changes were minimized; other than the text becoming 2-line strings when necessary, the only other visible change is that the album art frame is a bit larger than it was before, and is now centered in its space.
Video of new string layout behavior as main window is resized: