Fix handling of single letter shares in UNC paths in wxFileName

This comes at the price of breaking compatibility and returning
"\\share" rather than just "share" from wxFileName::GetVolume() for the
UNC paths. This breakage seems justified because it is required in order
to allow application code to distinguish between paths "x:\foo" and
"\\x\foo", which was previously impossible as GetVolume() returned just
"x" in both cases.

Document this change, adjust the existing checks for the new GetVolume()
semantics and add a new test which passes now, but didn't pass before.

Closes #19255.

This commit is best viewed ignoring whitespace-only changes.

docs/changes.txt

@@ -133,6 +133,12 @@ Changes in behaviour not resulting in compilation errors
   bitmaps in wxMSW if the corresponding Set had been called before, as in the
   other ports, instead of returning the normal bitmap as fallback in this case.
 
+- wxFileName::GetVolume() now returns "\\share" and not just "share" for the
+  UNC paths (i.e. \\share\path\on\remote\server) and "\\?\Volume{GUID}" for the
+  volume GUID paths rather than just "Volume{GUID}" as before. This allows
+  distinguishing them from the drive letters, even for single letter network
+  share name.
+
 
 Changes in behaviour which may result in build errors
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