Never leave a collection folder without a name, and say when it is broken

Fixes https://qelectrotech.org/bugtracker/view.php?id=332

localName() set a non-root folder's label only inside the success path of
loading its qet_directory file. If that load failed -- file missing,
malformed, or unopenable because of the Windows path-encoding problem with
accented characters that plc-user diagnosed on the tracker -- nothing was
set at all, and since a fresh item's text() is null the folder rendered
with a completely blank label. That is the reported symptom.

Resolve the name into a local and always fall back to the folder's own
directory name, so the label is never empty whatever went wrong.

The fallback is applied *after* NamesList::name() rather than passed into
it. This matters: name() returns a caller-supplied fallback before it
reaches its "first available translation" step, so passing m_path in would
replace a perfectly good name in some other language with the raw
directory name. A folder named only in French, viewed under an English
locale, previously showed "Accentué" and must keep doing so.

Falling back on its own would then hide the broken file -- the user sees a
plausible name and never learns there is anything to repair. So a folder
whose qet_directory could not be read now says so in its tooltip, naming
the file, above the collection path that tooltip already carried.
Suggested by plc-user on PR #622. The flag is recorded in localName() and
consumed in setUpData(), because setUpData() assigns the tooltip after
localName() runs and would otherwise discard it.

Only a file-level failure is flagged. A readable qet-directory with no
entry for the current language is not an error; NamesList::name() resolves
that itself and no warning is shown.

Verified on a fixture collection of four folders -- valid, malformed,
missing, and one named only in French:

                  master        this patch
  fr-only         Accentué      Accentué        (no warning)
  malformed       <blank>       malformed       (warning)
  no qet_directory <blank>      no_file         (warning)
  valid           Valid Folder  Valid Folder    (no warning)

sources/ElementsCollection/fileelementcollectionitem.h

@@ -64,6 +64,11 @@ class FileElementCollectionItem : public ElementCollectionItem
 
 	private:
 		QString m_path;
+			/// True when this directory's qet_directory file is missing or
+			/// unreadable, so setUpData() can say so in the tooltip. Recorded
+			/// rather than acted on in localName(), because setUpData() resets
+			/// the tooltip afterwards and would otherwise discard it.
+		bool m_qet_directory_unreadable = false;
 };
 
 #endif // FILEELEMENTCOLLECTIONITEM2_H