fix: Support Header elements within Title elements

[SpraxDev]

Description

We encountered issues with a Word document where this is a valid combination. This fixes parsing the document properly.

~~Fixes # (issue)~~ No open issue found (but similar to #2616)

Checklist:

• [x] My CI is :green_circle:
• [x] I have covered by unit tests my new code (check build/coverage for coverage report)
• [x] ~~I have updated the documentation to describe the changes~~ Does not apply
• [x] I have updated the changelog

[SpraxDev]

Is a test case needed for this kind of change? Do you have a recommendation on where to put it?

[coveralls]

coverage: 96.95%. remained the same when pulling 890f6558e53ee5be54efd816e0828fe0b14cb405 on SpraxDev:fix/support-header-within-title into 136f5499fc2ada425f5742127683af6f336cc309 on PHPOffice:master.

[SpraxDev]

Sure @Progi1984, I've updated the changelog and rebased the branch ^^

[Progi1984]

Thanks @SpraxDev for your contribution.