This just runs:
for i in *.[ch]; do
 sed -i s/DisplayX11/DisplayBroadway/g $i;
 sed -i s/PrivateX11/PrivateBroadway/g $i;
 sed -i s/ImplX11/ImplBroadway/g $i;
 sed -i s/KeymapX11/KeymapBroadway/g $i;
 sed -i s/ScreenX11/ScreenBroadway/g $i;
 sed -i s/GdkX11Monitor/GdkBroadwayMonitor/g $i;
 sed -i s/_X11/_BROADWAY/g $i;
 sed -i s/x11/broadway/g $i;
done