Add schema validation for scxml documents

So far, we don't validate the SCXML by XML schema at all before generating code from it. This is unfortunate because we cannot be sure that we don't accept invalid scxml that will then blow up in some other place in the user's toolchain.

In the bootstrap library we don't have a way to validate XML by schema, so it will be hard to do this in qscxmlc (as long as the build system is not overhauled to provide a fully featured host Qt). However, for runtime-loaded scxml we could provide a schema validation using QtXmlPatterns (optionally, if that is available).