feat: register images from paste + HTML insertion, fixes #790

[johanneswilm]

This takes care of images inserted by means of insertContent, pasting or potentially other means. Note: Images with a src-attribute linking to an image on another website will run into CORS issues. This is handled gracefully, but it means that these images will not actually end up in the document. Base64 encoded images should work fine.

[artem-harbour]

Added a couple of comments for consideration.

Note: also need to pay extra attention to adding new appendTransaction so that it doesn't lead to infinite loops and affect performance.

fyi @harbournick @johanneswilm

[artem-harbour]

LGTM

cc @harbournick

[johanneswilm]

Added a couple of comments for consideration.

I believe I answered all of those.

Note: also need to pay extra attention to adding new appendTransaction so that it doesn't lead to infinite loops and affect performance.

True, that is always important to consider. But as long as one has it limited to only acting in very specific cases and filtering out all other cases and not performing any dom lookups/operations before determining that another tr is needed, it should not have much of an impact on performance.

If one would want to limit the number of appendTransactions in the future, one could turn this into a single function that is executed to find all relevant content that enters through paste and the insertContent command. It could then make sense to move it to a clipboard handling plugin or similar.

[harbournick]

I moved everything to an internal PR and merged from there FYI: #1010
Closing this one. Just seemed quicker as I was testing and resolving merge conflicts. Feature looks great @johanneswilm !