Use full-height separators in ContentView

Summary:
The vertical separators currently end a few pixels short of the top and bottom,
creating a sense that the window is one big thing with three loosely separated
sections. This is rather visually muddy and not consistent with KDE's other
modern apps, which all use full-height lines to separate distinct areas of the
user interface.

This patch brings the same thing to Elisa, and substantially cleans up the code
pertaining to showing and hiding the separators.

Test Plan:
Album view, before:

Album view, after:

Now Playing view, before:

Now Playing view, after:

Reviewers: mgallien, Elisa, VDG

Reviewed By: mgallien, Elisa

Differential Revision: https://phabricator.kde.org/D21636