Implement Ignore Empty Paragraph feature

[Garry Yao]

After #3189 and #3190, empty paragraphs might get inserted in the document without user's intension, so clean up those empty blocks is necessary on output data.

[Frederico Caldeira Knabben]

We don't need really to remove all empty paragraphs. We're talking about the ​IgnoreEmptyParagraphValue setting we have in V2 today, which indicates that the editor value must be empty if we just have an empty paragraph inside of it.

Also, we have also some change on #3439.

I think this fix should go in the getData function of the wysiwygarea plugin, checking the data even before calling the data processor.

[Garry Yao]

Certain differences with v2:

1. We should consider all empty block as paragraph;
2. It's easier to have data processor to help with trimming empty inline elements, now we just need to consider spaces, &nbsp; and <br/>, later, we may even only need to consider spaces.

[Garry Yao]

Replying to garry.yao:

Certain differences with v2:

1. We should consider all empty block as paragraph;

No, the fixing should dedicate to paragraph-like block elements only.

[Frederico Caldeira Knabben]

The fix is ok. I would ask you to just move the regex declaration to the very beginning of the plugin code when committing, so it doesn't need to be recreated every time the inner function get executed.

[Garry Yao]

Fixed with [3645].