Make the "Show Playlist" action more obvious, and hide the playlist at small window sizes
In order to make the ability to show an hide the playlist more obvious, this MR moves the action from the hamburger menu and to the main toolbar. Then it also implements auto-hiding the playlist when the window size is narrow. When the window is widened, the playlist is automatically shown.
We gotta do something about that flickering in the HeaderBar...
Because it's now a toggle action on the toolbar, the global menu version is also made a toggle action to match it.
Keyboard shortcuts still work, tab navigation and triggering with the return key work fine.
BUG: 392778 FIXED-IN: 20.04.0