Support 3 areas for the buttons on the Source Editor toolbar

Currently, we provide two options for adding buttons to the Source Editor toolbar: left and right. Once we start using the toolbar for real (like Migrate actions from Markdown toolbar to Source... (#352662 - closed)), we will experience this as a limitation.

For example, we will figure out that the buttons proximity is going to be confusing: most of the buttons on the markdown toolbar will be related to formatting the content of a file (bold, italic, etc.). But there's also the button (not markdown-specific) to toggle wrapping of the text. This button doesn't belong to the same group of buttons as, let's say bold. Also, we have "Live Preview" button for the Markdown, which is also not formatting related, but is editor-wide action. Logically, No wrap and Live Preview buttons should be grouped into their own group separate from the formating buttons.

The current look of the toolbar with all the buttons migrated to the SE toolbar:

Why not use the left group then?

At some point in the near future we will be adding items to that group as well and those will be related to the multi-file experience in the first iteration. Which does not logically relate to the editor actions like preview or wrapping. Hence, the suggestion to introduce the third group of the buttons

How will it look and feel?

I was playing with this locally, and here's what I would suggest