Added multiply inherited class for PHP objects

[RobertoRoos]

Created extra classes for PHP objects, to combine Breathe's BaseObject with phpdomain's classes, much like was already done for C#. This improves on #217

I am not entirely certain how well it works now. It does work well on this tiny example: https://github.com/RobertoRoos/PHP-Breathe-Demo

It might be good to use it on a bigger project (with good documentation) to test the result.

[vermeeren]

@RobertoRoos The diff seems good to me, but the linter is complaining a bit. Can you check? Will merge and release version if all is good. Based upon location and similar code I think this is a false positive so you can just add the # type: ignore behind it. Note that this must be prefixed with a double space.

As for further testing: I personally don't know a larger project using PHP with Breathe, usage of Breathe this way is likely pretty niche considering how few issues and patches I see about it compared to the other languages. Most likely we'll see further issues and improvements, if needed, as time passes.

[vermeeren]

@RobertoRoos All good, thanks again! I intend to make a release after the remaining open PR is finished, probably within a week or so. Is that ok for you?

[RobertoRoos]

Yeah, that's fine. Thank you for the merge!