Post Selector: Add option to adjust taxonomies relationship

[Misplon]

This issue is an expansion of #752. The summary is to allow for a more advanced syntax in the Taxonomies field.

Add support the AND and OR relations directly in the field. I.e. the Taxonomies field would allow simple entries like

OR(category:quotes,post_format:post-format-quote)

which would be equivalent to what we have currently as

category:quotes,post_format:post-format-quote

and we could allow more complex nested queries like

OR(category:quotes,AND(post_format:post-format-quote,category:wisdom))

The general idea is to not add another field and just allow a slightly more complex syntax for advanced users.

Related forum thread: https://siteorigin.com/thread/syntax-for-requiring-multiple-categories-in-post-loop-taxonomy/. This comes up a couple of times a month.