Add wxUILocale::IsSupported()

This function can now be implemented relatively straightforwardly
(although it does require an extra check under Mac), so add it, as it
can be generally useful and we're also going to need it for our own
tests in the upcoming commit.

include/wx/uilocale.h

@@ -48,6 +48,10 @@ public:
     // Create the object corresponding to the given locale.
     explicit wxUILocale(const wxLocaleIdent& localeId);
 
+    // Check if the locale is actually supported by the current system: if it's
+    // not supported, the other functions will behave as for the "C" locale.
+    bool IsSupported() const;
+
     // Get the platform-dependent name of the current locale.
     wxString GetName() const;