Protoype to report/confirm overwrite of 'library' blendfiles.

This commit extends the reporting and overwrite confirmation system
created for blendfiles saved by later Blender version, to also detect
and handle 'library' blendfiles (typically saved by partial-write API,
they do not have a defined current Screen or Scene in their global
data).

the implementation details are fairly straightforward, but this commit
raises some design questions:

* Do we care reporting in the status bar about the 'library blendfile'
  case? Unlike opening a more recent blender version blendfile, opening
  a library file has no loss of data.
* When both issues are present (and potentially more in the future), do
  we want a merged info of all present issues, or do we define some sort
  of priority order, and only report the highest priority issue?

As it is right now, this commit can make both the status bar tooltip,
and the overwrite confirmation popup, fairly busy...

Obviously, naming and UI messages are also open for discussion.

@@ -6561,0 +6584,4 @@
+      tooltip_message += "\n\n";
+    }
+    tooltip_message += RPT_(
+        "This is an Asset Library Blender file, overwriting it\nmay break its handling by the "