Improve support for gtk-csd-reversed-placement

This patch changes how we deal with window control buttons on left
layouts. Previously, we looked to a pref that was only set on macos
but with this patch we use media queries to detect whether the system
is macOS or gtk-csd buttons use left-side layout.

To support old-style "override pref" this patch also introduces a
new custom pref userchrome.force-window-controls-on-left.enabled
which can be used e.g on Windows to move window controls to left side.
This logic is currently only supported in styles that use
window_control_placeholder_support.css

chrome/overlay_menubar.css

@@ -32,6 +32,6 @@ See the above repository for updates as well as full license text. */
   background-image: linear-gradient(to right, var(--arrowpanel-background) 30%, transparent);
 }
 
-@supports -moz-bool-pref("layout.css.osx-font-smoothing.enabled"){
+@media (-moz-gtk-csd-reversed-placement){
   #toolbar-menubar[autohide="true"]:not([inactive]) > .titlebar-buttonbox-container{ display: none }  
-}
+}