make block position definable in the list of blocks #185

[PatricB]

Describe the PR

So far, custom blocks are simply added at the end of the standard blocks. In some cases, however, it is practical to place your own blocks at a defined position (e.g. en h4 block after the h3 block).

Related issues

Sort blockTypes #185

Ready?

• [ ] Added unit tests for fixed bug/feature
• [ ] Passing all unit tests
• [ ] Fixed code style issues with CS fixer and composer fix
• [x] Added in-code documentation (if needed)