UV: support changing the opacity of the UV overlay

Add this option as it's useful to adjust how much UV's
cover the image when UV mapping.

D5348 by @EitanSomething with edits

release/scripts/startup/bl_ui/space_image.py

@@ -955,6 +955,7 @@ class IMAGE_PT_view_display_uv_edit_overlays(Panel):
         col = layout.column()
         col.prop(uvedit, "show_smooth_edges", text="Smooth")
         col.prop(uvedit, "show_modified_edges", text="Modified")
+        col.prop(uvedit, "uv_opacity")
 
 
 class IMAGE_PT_view_display_uv_edit_overlays_stretch(Panel):