Improve UI consistency between PlaylistView and NavigationActionBar
To make the album icon, text and margins in Media/SimplePlaylistView consistent with the ones in NavigationActionBar, we change the coverArtSize to Kirigami.iconSizes.medium (same which is used in Kirigami.Icon) instead of unitSpacing and the ListSectionHeader level (font size) and margin to match.
Before:
After:
