Fix wxURI::Unescape() to work with Unicode strings

Such strings are not really URIs as they should have been encoded if they were
but we can obtain them from e.g. wxFileSystem::FindFirst(), so handle them
correctly here as it's simpler than checking all the places where Unescape()
is called.

Add a unit test checking that decoding an URI containing both Unicode and
percent-encoded Unicode characters works correctly.

tests/uris/uris.cpp

@@ -338,6 +338,15 @@ void URITestCase::Unescaping()
                             "\xD1\x87\xD0\xB8\xD1\x81\xD0\xBB\xD0\xBE"
                           ),
                           unescaped );
+
+    escaped = L"file://\u043C\u043E\u0439%5C%d1%84%d0%b0%d0%b9%d0%bb";
+    unescaped = wxURI::Unescape(escaped);
+
+    CPPUNIT_ASSERT_EQUAL
+    (
+        L"file://\u043C\u043E\u0439\\\u0444\u0430\u0439\u043B",
+        unescaped
+    );
 #endif // wxUSE_UNICODE
 }