Make Clipboard plugin required by ImageUploadEditing.

[jodator]

Suggested merge commit message (convention)

Other: Make Clipboard plugin required dependency of ImageUploadEding. Closes ckeditor/ckeditor5#2910.

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Additional information

• This is the only required fix I've found for 'soft' requires in CKEditor5 code base.
• ps.: I didn't add the BREAKING CHANGE: entry as I don't see this a breaking change but might be needed from a puristic POV:

BREAKING CHANGE: The ImageUploadEding now requires Clipboard plugin to work.

• also the BallonToolbar usage is OK:

About that BalloonToolbar plugin use - it is OK. It is not a plugin but a class of and BalloonEditor which always requires BalloonToolbar.

[coveralls]

Coverage decreased (-0.0009%) to 99.704% when pulling daf0741 on t/ckeditor5-core/193 into 2cfe305 on master.

[Reinmar]

Some issue with coverage.

[jodator]

It is because of number of lines have decreased and we still didn't fix coverage here.