GPencil: Add an xray toggle for each annotation layer

so a layer can be occluded by the scene instead of always showing in front

---

{F12827163}

Reviewed By: fclem, antoniov

Differential Revision: https://developer.blender.org/D13931

release/scripts/startup/bl_ui/properties_grease_pencil_common.py

@@ -361,6 +361,10 @@ class GPENCIL_UL_annotation_layer(UIList):
             split.prop(gpl, "info", text="", emboss=False)
 
             row = layout.row(align=True)
+
+            icon_xray = "XRAY" if gpl.show_in_front else "FACESEL"
+            row.prop(gpl, "show_in_front", text="", icon=icon_xray, emboss=False)
+
             row.prop(gpl, "annotation_hide", text="", emboss=False)
         elif self.layout_type == 'GRID':
             layout.alignment = 'CENTER'