Fix elided text in playlist headers and clean code and design somewhat

Nate Graham requested to merge fix-elided-text-in-playlist-header into master

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.

Before: Before

After: After

Video of new string layout behavior as main window is resized: demo-2019-12-31_16.16.27

BUG: 406470

FIXED-IN: 20.04.0

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