Tests: changed incorrect names: plain/html -> text/html.
Tests: Improved mockNativeDataTransfer.
Tests: for #12341.TC1.
Fixed Copy text form textarea (#12173.TC1).
Delete content with the low priority.
Fixed docs for initPasteDataTransfer and initDropDataTransfer.
Changed CKEDITOR.DATA_TRANSFER_INTERNAL... flags ids so they start from 1.
Introduced canListenOnDocument.
Introduced isPasteEventFreelyAvailable, isCustomDataTypesSupported and getDropTarget.

Conflicts:

	plugins/clipboard/plugin.js
	tests/plugins/clipboard/datatransfer.js
Renamed isPasteEventFreelyAvailable to isDataFreelyAvailableInPasteEvent.
Merge branch 't/12341' into major