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.

tests/intl/intltest.cpp

@@ -240,6 +240,13 @@ TEST_CASE("wxLocale::Default", "[locale]")
 
 #endif // wxUSE_UNICODE
 
+TEST_CASE("wxUILocale::IsSupported", "[uilocale]")
+{
+    CHECK( wxUILocale("en").IsSupported() );
+    CHECK( wxUILocale(wxLocaleIdent("fr").Region("FR")).IsSupported() );
+    CHECK( !wxUILocale("bloordyblop").IsSupported() );
+}
+
 TEST_CASE("wxUILocale::GetInfo", "[uilocale]")
 {
     CHECK( wxUILocale("en").GetInfo(wxLOCALE_DECIMAL_POINT) == "." );