Support for pasting images

[f1ames]

Suggested merge commit message (convention)

Feature: Support for pasting content with images from Word. Closes ckeditor/ckeditor5#2515. Closes ckeditor/ckeditor5#2529. Closes ckeditor/ckeditor5#2534.

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Additional information

This PR implements two mechanisms - one for handling images in Chrome, Firefox and Edge and one for handling images in Safari.

Chrome, Firefox, Edge

When content is pasted from Word, images are represented wit local sources file://. In the browser clipboard apart from text/html data, there is text/rtf data provided which contains entire images encoded as hex strings. These strings are extracted, converted to base64 and used as new image elements sources e.g <img src="data:image/jpeg;base64,..." >.

This is a synchronous process so all content manipulation takes place on a view instance so data inserted on paste is in its final form (no transformations are applied after data is rendered in the editor).

Safari

When content is pasted from Word, images are represented with local blob resource blob://. Such resources are then fetched (via XHR), converted to base64 representation and then used as new image elements sources e.g <img src="data:image/jpeg;base64,..." >.

As fetching blobs is an asynchronous process, first the data with blobs is inserted into the editor (images are already visible) and then when each blob is fetched and converted, images sources are replaced.

Note: As this PR was built on top of t/12 (as it provides whole infrastructure for automatic testing on Safari), it can be merged only after #16 (t/12 PR) is merged.

[coveralls]

Pull Request Test Coverage Report for Build 148

• 79 of 79 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 141:	0.0%
Covered Lines:	163
Relevant Lines:	163

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💛 - Coveralls

[f1ames]

Ready for review 🎉

[Mgsy]

Everything works fine 👌

[pjasiun]

Make sure that, if the image upload adapter is provided, no base64 data will be inserted in the model. In would break collaboration. Also, ensure that the upload progress displays properly on other collaboration clients (or report a follow-up about it if there is any problem with it).

[f1ames]

I have removed logic responsible for blob extraction (moving it to UploadImage plugin). The snapshot of the branch in the previous state (with blob handling) is available under t/21-b.

[f1ames]

Just to clarify the previous comment. Moving blob extraction to UploadImage plugin means that without the UploadImage plugin, images pasted from Word in Safari will be represented as blobs and will not be converted to base64. We have agreed with @Reinmar that this is acceptable solution for now, but may also think about improvements in the future if needed.

[f1ames]

This PR contains also some additional space normalization which apparently solves #39 and #40, I have added to Closes commit message.

[Reinmar]

Haha :D While checking the code I thought... shouldn't it fix the issue that @f1ames just reported? :D

[f1ames]

😄 👍

[Reinmar]

I've done some testing and it works as expected. I'm going to have a quick look at the code still (especially in ckeditor/ckeditor5-image#248) but we should be able to merge this very soon.

[Reinmar]

Comments.