GPencil UI: Make "Tint" settings take full width of panel in 2D editors

Parenting options are not visible there (i.e. they're only for the 3D view),
so reserving space that isn't going to be used in those editors doesn't really
make much sense. Furthermore, those property regions are often quite narrow
too, so it doesn't help too much to keep these settings so narrow there.

release/scripts/startup/bl_ui/properties_grease_pencil_common.py

@@ -834,7 +834,10 @@ class GreasePencilDataPanel:
         split.prop(gpl, "show_points")
 
         # Offsets + Parenting (where available)
-        split = layout.split(percentage=0.5)
+        if context.space_data.type == 'VIEW_3D':
+            split = layout.split(percentage=0.5)
+        else:
+            split = layout.column()  # parenting is not available in 2D editors...
         split.active = not gpl.lock
 
         # Offsets - Color Tint