Collapsible Block: Add a way to "pin" a block open temporarily to work on the content

[acketon]

Currently the Collapsible Block shows the content area when it is selected allowing you to enter content into the "collapsed" content area of the block. However if you deselect the block it is immediately hidden.

When a lot of content is being put into the collapsible block it would be a better editing experience to be able to "pin" the block open just for that editing session and keep it open regardless if you are clicked into it. The pinned state should either only effect the editor view or reset when the editor is refreshed.