groupStart() refactorization

[michalsn]

Description
Since we have now groupStart() and havingGroupStart() it's a good moment to refactor the code responsible for those methods a bit.

In this PR I want to introduce two new protected methods: groupStartPrepare() and groupEndPrepare(). These will take over the responsibility for handling all other group* methods.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[jim-parry]

It even looks like the user guide doesn't need updating ... groupStart() there doesn't show parameters :-/
Are you planning unit tests for this?
I don't see this PR as urgent as 2269, as there doesn't appear to be any BC here. Is that right?

[lonnieezell]

I like this. As long as we have existing tests in place for the groups then this should be good and doesn't need any additional tests.

[michalsn]

The parameters for groupStart() were marked in code as "internal use only" so we should be good - no BC changes.

We have tests for existing group methods and I think it should be enough.