[Umbrella] Rich content drag-and-drop and files pasting and dropping support

[Piotr Jasiun]

Umbrella ticket for drag-and-drop and direct pasting of files for CKEditor.

Parts:

• #11526 (research)
• #11460 (handling dropped content)
• #11580 (notifications plugin)
• #11461 (handling pasted and dropped images, upload manager)
• #11219 (aligning widgets dnd to changes introduced by #11460)
• #11636 (improve selection.getSelectedHTML, range.cloneContents and range.deleteContents).
• #11975 (tables).
• #11976 (lists).
• #11977 (images).
• #11978 (multiselection, might be postponed).
• #11979 (ctrl key, might be postponed).
• #12090 (reusing global data transfer in block widgets dnd).
• #12091 (editor#dragenter).
• #12095 (dnd inline widget loses styles (e.g. bold))

Note: each of these parts should be handled in its ticket, not in this umbrella ticket.

Related: #7422.

Edit: Small test case to check #11657. It seems that in inline and divarea editors there is something wrong with selection and D&D.

[Piotr Jasiun]

I talked with Fred and we will focused on images for now.

[Wim Leers]

Impressive work!

[Piotrek Koszuliński]

As you can see in #11526 and #11460 the feature turned out to be a very demanding challenge, the design phase took nearly all the time we had for implementation phase. We don't want to postpone CKEditor 4.4 for which we have enough new features, so we're rescheduling drag and drop instead.

[Frederico Caldeira Knabben]

While playing with the manual testing page in t/11460, I had this issue:

1. In the classic editor, start a selection before "11" in the title up to the end of "spaceflight" in the first paragraph.
2. Drag the selection after "landed" in the first paragraph.

The dragged content will be lost and no insertion happens. A js error is thrown.

[Piotr Jasiun]

During the previous weeks custom D&D was deeply testes and about 30 issues were found:

​https://docs.google.com/document/d/1hG4H0r21MXNkRd3amDEOBPygJe3ehBAXFWAal2DptGQ/edit?usp=sharing

After considering this results the future plan is to:

• fix most important part of bugs as a part of #11460,
• improve getSelectedHTML, cloneContents and deleteContents (#11636), because this is the reason of some bugs (ex. Bug 4, Bug 7),
• fix D&D for tables (#11975), lists (#11976), images (#11977) and eventually Firefox multiselection (#11978) after fixing #11636,
• leave part of the issues as separate tickets (#11979) or close as "won't fix" (ex. Bug 6).