Validating branch mess

[leamas]

Currently, validating new PR:s is done with xsd files etc. from the master branch, even if the PR is for Beta. This creates an impossible situation when the set of allowed ABI:s differs between the Beta branch and master. One example is #815.

For example, in current cycle we have updated the specifications on master branch to match the needs of the Beta/5.7/5.8. This causes problems when validating PR for current stable 5.6.2.

In a better world we should use references like ocpn-plugin.xsd from Beta branch when validating Beta, merging Beta into master when 5.8 is released. This would avoid disturbing updates to stable/master while working with Beta