ACF callback to prevent filtering on matching element

[Piotrek Koszuliński]

Required for #10925.

Explained in #11503.

Disabling ACF without a possibility to communicate through element's attributes. In this case the only possible solution is a callback executed by ACF which will notify it about how it should proceed. For example mediaembed will add a callback which will tell ACF that all descendants of element should be skipped if element has mediaembed's class. Callback will return binary flags, because we may need more options in the future. Example of flags that we may need to implement:

• DISABLE_ON_ELEMENT - skips only this element (continues on descendants),
• DISABLE_ON_DESCENDANTS - skips descendants,
• CONTINUE_ITERATION - continue iteration - used with DISABLE_ON_DESCENDANTS could inform ACF that it should look for a descendant that reenables filtering,
• ENABLE - reenable filtering.

But of course for now we'll need just one of these flags, so only one should be implemented.

[Piotrek Koszuliński]

Pushed ​branch:t/11737 on dev and tests.

[Olek Nowodziński]

Rebased dev branch on latest major.

[Piotrek Koszuliński]

Merged to major with ​git:58af051 on dev and fdff52b on tests.