Add an option to make widgets non-removable

[xmo]

On the new widgets feature, it's currently possible to focus and delete a widget.

Depending on the situation, this may not be a desirable behavior in all situations, e.g. the editable may not make sense without the widget, although editing both the content and surrounding (text) of the widget makes sense.

See attachment for an example, set caret at the start of the last paragraph then backspace twice, the widget (middle paragraph) will first be selected/focused, then removed. This may be preventable by catching the deletion event at the editor level and stopping it, but that feels hackish.

[Piotrek Koszuliński]

The thing is that there are many ways in which widget could be removed. New page button, deleting widget itself, deleting bigger text selection, cutting, replacing with other content when pasting/inserting other things, etc, etc.

I understand the need for such feature, but it's so unfeasible that I guess we'll never work on it.