Adjust settings panel to autosize to zero when hiding it

Previously, when hiding the settings overlay, it remains to have a width
of `56` (sidebar width), this is due to the panel content being placed
next to the sidebar, so therefore the content has to move 400
(PANEL_WIDTH) + 56 (sidebar_width) backwards, for the overlay to have a
width of 0 on hide.

osu.Game.Tests/Visual/Settings/TestSceneTabletSettings.cs

@@ -27,7 +27,7 @@ namespace osu.Game.Tests.Visual.Settings
                 new TabletSettings(tabletHandler)
                 {
                     RelativeSizeAxes = Axes.None,
-                    Width = SettingsPanel.WIDTH,
+                    Width = SettingsPanel.PANEL_WIDTH,
                     Anchor = Anchor.TopCentre,
                     Origin = Anchor.TopCentre,
                 }