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
