T/ckeditor5/1555: Properly handle enter key on objects.

[jodator]

Suggested merge commit message (convention)

Fix: Editor crashes after enter key on an image that is inside a blockquote. Closes ckeditor/ckeditor5#1555.

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Additional information

• This PR handles properly (as described in comment before if) enter key on object elements. The previous fix introduced problem with objects inside elements that were not described as limit in schema (<blockquote> is nether block nor limit nor anything else). The comment wasn't inline with the code as code checked is the element is not an limit rather then if it is a block (a block can be an object thus a limit).
• The better would be just to ignore inline elements in that handler as they should be treated as text - so the inline object should be removed by that handler.
• Also I found the superfluous check for delete key check in detecting direction of an arrow in arrow keys handler code which was removed.

[jodator]

@Mgsy could you briefly check that? I think that every case (including shift+enter) is handled - in root or inside a table cell also. This touched the inline-widgets also.

[Mgsy]

LGTM 👍