Simplify insertion flow

[jasmussen]

After discussions with @mtias (#798 (comment)) and @aduth and seeing some of the complexity that the proposed flow for inserting (#323) has presented, perhaps we should think of a simpler flow for insertion.

To be absolutely clear — the purpose of these mockups is to make the code simpler too. If these ideas and concepts add complexity, please do let me know. They are not gospel, and your thoughts are important here.

Flow

The inserter lives in the Editor bar at the top, and at the bottom of the post for quick access near where you're writing:

Click the inserter to open the block tray. Hovering a block in the inserter shows a blue line where the block will be inserted. The block will always be inserted after the selected block, and if nothing is selected, at the end of the document:

In the above mockup, the first two paragraphs constitute a single text block, and the last paragraph is also a separate text block. Since the caret is in the first text block, the image about to be inserted will show up after.

CC: @aduth @youknowriad — will this make for a simpler, codewise, flow?

[youknowriad]

@jasmussen This looks good to me and simple enough in terms of "code". I'm going to try to implement it.

[mtias]

Agreed this is a much simpler flow code wise, since we only deal with block ordering. The selection state is slightly related to: #827.