Fix for cutting in Safari when selecting all with widget at the beginning

[Comandeer]

What is the purpose of this pull request?

Bug fix

Does your PR contain necessary tests?

All patches which change the editor code must include tests. You can always read more
on PR testing,
how to set the testing environment and
how to create tests
in the official CKEditor documentation.

This PR contains

• Unit tests
• Manual tests

What is the proposed changelog entry for this pull request?

* [#3537](https://github.com/ckeditor/ckeditor4/issues/3537): [Safari] Fixed: can't cut when the whole content is selected and widget is at the beginning of it.

What changes did you make?

I've manually added widgetselection fillers just for Safari.

Closes #3537.

[f1ames]

Rebased onto latest master.

[f1ames]

There is one issue with the current solution, after cutting there is additional empty paragraph (<p></p>) left in the content. When you paste back cut content, you are not able to select all again because of the <p></p> at the content end. It also pollutes getData(), etc.
It looks like a little broader issue, related to getClipboardHtml(), but still it worked before on Safari (select all + cut) so should be investigated.

[Comandeer]

It seems like a deeper issue. I've extracted it to #3705.

[f1ames]

#3705 was merged to major branch so we can restore work on this PR 🎉

[Comandeer]

I've rebased the branch onto latest major, but the issue with additional, empty p is still present 🤔. Seems like there is some other source of the issue.

[Comandeer]

The issue is connected with the filler itself. Safari can't delete it properly after cutting, leaving its content (space) inside the editor. Editor converts it into the paragraph, which results in two paragraphs (one from the filler and default one, created automatically after cutting).

It seems like our filler needs its own filler to be deleted 😐

[f1ames]

Widgetselection plugin is quite complicate and so we could try to look for some improvements there. The first thing to check will be trying to simplify it according to issue reported some time ago - #1670.

If we could use the solution proposed there, the issue should go away and we will be able to proceed with this PR.