Move hide preview button to line between composer and preview

[jomaxro (Joshua Rosenfeld)]

@codinghorror

And adding a dynamic preview resize bar is out of scope for what we’re doing here, that would be a fairly major feature to add, that we’ve never had.

I like @barryvan’s suggestion. I don’t believe he was suggesting a resizable preview area, just replacing the text “hide preview” with a button in the middle. Could be grey/black if blue is too much.