LibOverride: Expose the 'needs resync' status of libraries to RNA/Py API.

Could be useful for automated update/sanitizer tools in a production
pipeline.

Note that this propoerty is left editable for now, on purpose (external
tools may want to clear the flag).

source/blender/makesrna/intern/rna_ID.cc

@@ -2493,6 +2493,14 @@ static void rna_def_library(BlenderRNA *brna)
   RNA_def_property_clear_flag(prop, PROP_EDITABLE);
   RNA_def_property_flag(prop, PROP_THICK_WRAP);
 
+  prop = RNA_def_property(srna, "needs_liboverride_resync", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(prop, nullptr, "tag", LIBRARY_TAG_RESYNC_REQUIRED);
+  RNA_def_property_ui_text(prop,
+                           "Library Overrides Need resync",
+                           "True if this library contains library overrides that are linked in "
+                           "current blendfile, and that had to be recursively resynced on load "
+                           "(it is recommended to open and re-save that library blendfile then)");
+
   func = RNA_def_function(srna, "reload", "rna_Library_reload");
   RNA_def_function_flag(func, FUNC_USE_REPORTS | FUNC_USE_CONTEXT);
   RNA_def_function_ui_description(func, "Reload this library and all its linked data-blocks");