Fix #120574: Broken animation when saving file with NLA tweak mode entered

Fix linking & library-overriding with NLA Tweak Mode enabled. This is a
two-pronged approach:

- When linking an animated ID from a library file, and it happens to be
  in NLA Tweak Mode, it is forced out of tweak mode. This ensures that
  the correct Action is loaded, and that all the NLA flags are set
  correctly to display the NLA animation (instead of only the tweaked
  strip).
- For library overrides there is now a post-process step that runs after
  all 'apply' functions have been run. This is necessary to ensure that
  all the flags and pointers that NLA Tweak Mode depends on are actually
  set correctly and consistently.

This also adds one utility function `BKE_nla_debug_print_flags()` that
is by now unused. It was very useful in debugging this, though, and I
think it'll be useful in the future as well.

Design task: #120573

@@ -5046,6 +5046,8 @@ void BKE_lib_override_library_update(Main *bmain, ID *local)
     }
   }
 
+  BKE_animdata_liboverride_post_process(local);

@@ -2447,0 +2589,4 @@
+  }
+
+  /* In tweak mode, with tracks, so ensure that the active track/strip pointers are correct. */
+  if (!adt->act_track || !adt->actstrip) {