Draft: frames: Use libadwaita to style legacy X11 windows
Styles legacy X11 windows that use server side decorations using
libadwaita, to make them feel less out of place. Note that since
Mutter is also used by Pantheon, which does not use libadwaita
styling, this functionality is enabled conditionally. If
XDG_CURRENT_DESKTOP
is set to GNOME
or MUTTER_USE_ADWAITA
is
set to 1
.
Additionally, this adds a style class, ssd-frame
to the
GtkHeaderBar
that is parented by MetaFrameHeader
, so that
libadwaita can properly target it for styling. It needs this in order
to add a line that divides the windows headerbar from its content.
Fixes: #2830