Add support for branch/tag comparators in when block

[Alex-Weatherhead]

This pull-requests adds support for providing comparators for branch and tag filters in when blocks. As part of this work, functionality from the ChangeRequestDeclaration was separated out into the parent class (WhenDeclaration) to ensure that the approach was consistent with the similar work that had been done there.

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Fixes #452.

[BoydL]

Can this be looked into I am experiencing an issue with using pattern matching for my tags

[nre-ableton]

I'm quite clueless about the declarative API and tend to stay away from issues relating to that in this repository, but you seem to at least have provided some tests etc. Beyond that, I'm unable to give a thorough review to say if something has been overlooked or missed.