Use smarter position of the insertion

[oleq]

The media widget is inserted always after the block in which you currently have the caret. The image insertion works a bit better and if you have the caret at the beginning of that block it gets inserter before it. This can be a followup, of course.

A follow-up of ckeditor/ckeditor5-media-embed#2 (comment).

[pjasiun]

Similarly to my comments in the table feature (https://github.com/ckeditor/ckeditor5-table/issues/27#issuecomment-419917100 and https://github.com/ckeditor/ckeditor5-table/issues/27#issuecomment-419917623), it looks very inconsistent in Letters now.

Especially:

• when the position is at the beginning of the block, it should be inserted before the block,
• when the position is in the empty block it should be inserted instead of block.

This is now it looks like now:

[Reinmar]

Let's continue this discussion in #1243.