Fixes after #12173 review.

[Piotr Jasiun]

Part of #11437. Related to #12173.

• On editable#cut content should be deleted with lower priority (999). Currently it's hard to override cut data, because it's immediately gone.
• initPasteDataTransfer has no documentation.
• In initPasteDataTransfer and initDropDataTransfer you expect first argument to be what? Because docs says it's native event, when it's not.
• I mentioned this during one of previous reviews and I noticed this again - we can't write conditional code based on repeating CKEDITOR.env.ie usage. This makes code hard to read and hard to maintain, because it's unclear what this condition means - why IE, why not other browsers, whether IE12, 13 too or not. You need to create meaningfull variables like dataTransferSetCustomDataSupport and use them. E.g. in pasteDataFromClipboard you created htmlAlwaysInDataTransfer - it's great, just make it global for this plugin and then create more of them.
• CKEDITOR.DATA_TRANSFER_INTERNAL flags should start from 1 (1, 2, 4).