Allow creating wxUILocale objects for any locale

Creating such objects (without using them for the UI) is supported under
all platforms, so allow doing it.

Note that this is only supported under Unix systems when locale_t and
related functionality is available, but this should be the case just
about everywhere by now.

Add a test (or, rather, replace an existing test which was disabled by
default) checking that we can now get locale information about any
locale, not necessarily the currently used one.

interface/wx/uilocale.h

@@ -94,6 +94,21 @@ public:
      */
     static const wxUILocale& GetCurrent();
 
+    /**
+        Creates the locale corresponding to the given locale identifier.
+
+        In the simplest case, this can be used as following:
+        @code
+            const wxUILocale loc("fr");
+        @endcode
+        but more precise locale identifiers can be used, see wxLocaleIdent
+        description for more details.
+
+        If @a localeId is not recognized or not supported, default ("C") locale
+        is used instead.
+     */
+    explicit wxUILocale(const wxLocaleIdent& localeId);
+
     /**
         Compares two strings using comparison rules of the given locale.