Widget option for "always open".

Problem/Motivation

Since #2650306: Always offer Collapse/Preview/Edit → , every paragraph widget has the collapse option.
In comment 23 of that issue → , I argue that this is not always the right thing.
There should be an option to hide the collapse option and keep it always open.

Steps to reproduce

Proposed resolution

From the comment #23 of the other issue:

I think the best option would be to have 4 options:

• Always open (NEW)
• Open, collapsible
• Collapsed
• Collapsed, show nested

Or a checkbox "Allows expand / collapse" that would be enabled by default. Unchecking this would hide all of the options.

I would do this by subclassing the widget, but I am already doing this for other purposes. Too many dimensions of inheritance are not good..

Remaining tasks

User interface changes

API changes

Data model changes