[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)

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.