Less extreme paste as plain text - keep semantic but strip styles

[Piotrek Koszuliński]

In the current situation we have the ACF and forcePasteAsPlainText (forcePAPT). The ACF works perfectly unless following happen:

1. Editor has so many features enabled (e.g. fonts and colors) that it does not filter pasted content as precisely as user would want.
2. Developer decided to disable ACF because of complexity of content which is going to be edited.
3. Developer makes a decision that paste content should be filtered more strictly than content being edited. In this case he can use forcePAPT, but this option is too strict for many cases. For example one may want to preserve semantic, but no styling. See e.g. #11619.

The proposed solution is to replace forcePAPT with a separate ACF instance, which can be configured independently from the main one. We could have some presets available, like "semantic" preset which would allow HTML tags and some semantic attributes and "plaintext" preset which would allow only paragraphs and line breaks.

Note: these presets do not have to be correlated with editor features like main ACF is, because main ACF would be still applied to the inserted content.