Clarify `contenteditable="false"` elements which is associated with an `EditContext`

[masayuki-nakano]

https://w3c.github.io/edit-context/#edit-context-differences

Each child node of the EditContext editing host becomes editable, unless that node has a contenteditable attribute set to "false".

I guess that if the element whose contenteditable is "false" has an EditContext, it should be editable because if only the element has focus, it should be editable.

So in my understanding an element can be editable if:

• designMode is set to on
• contenteditable is set to true
• an EditContext is associated
• an ancestor is editable and not in designMode and not associated with an EditContext and contenteditable is set to "false"

I.e., an element in an editing host can be non-editable if and only if:

• not in designMode
• not associated with an EditContext
• contenteditable is set to "false"
• or a descendant of the non-editable element and is not an editing host

Cc: @dandclark

[dandclark]

This description matches my understanding of things should work. We do still need to discuss the designMode question in #74.

The example in this issue shows a lot of the interesting cases: #53

We also discussed in #38 the question of what happens if contenteditable=true and EditContext are set on the same element. The decision at the time was that the EditContext behavior wins over the contenteditable behavior. I think that should also be the case for contenteditable=false, so if contenteditable=false and EditContext are set on the same element, the element is editable and the contenteditable=false is ignored.