Send event when clearing wxTextCtrl in wxGTK again

This was broken by 1c946a469a and resulted
in test failures in OwnerDrawnComboBoxTestCase::TextChangeEvents() unit
test because wxOwnerDrawnComboBox::Clear() used SetValue("") and didn't
generate any events any more.

Fix this regression by explicitly sending an event if we're returning
early and add a unit test explicitly checking that SetValue("") does
generate an event.

tests/controls/textentrytest.cpp

@@ -57,6 +57,10 @@ void TextEntryTestCase::TextChangeEvents()
     CPPUNIT_ASSERT_EQUAL( 1, updated.GetCount() );
     updated.Clear();
 
+    entry->SetValue("");
+    CPPUNIT_ASSERT_EQUAL( 1, updated.GetCount() );
+    updated.Clear();
+
     entry->ChangeValue("bar");
     CPPUNIT_ASSERT_EQUAL( 0, updated.GetCount() );