[bv-dev] Support defining container element type contraints in XML

Hi, We made some progress regarding $SUBJECT (e.g. defining the @NotNull constraint in List<@NotNull String> property; in XML) and we ended up with the following PR: https://github.com/beanvalidation/beanvalidation-api/pull/94 I added an example of how it could be used for a nested example in the PR description. Feedback welcome! -- Guillaume