Heading plugin should be disabled when selection in the title

[oskarwrobel]

Follow-up: #2003

The document title is always represented as a <title /> element in the model. It's not possible to change it to the other block element (post-fixer will always fix such a change).

Commands that try to change title element should be disabled when selection is in the title.

[oskarwrobel]

In a case of elements that are not allowed to be a title (cannot be renamed to title), it works bad. E.g. after executing one of the list commands, title element is renamed to listItem then postfixer cannot rename it back to title because list cannot be transformed to title. In this case post-fixer creates an empty title before the list item.

<title>F[]oo</title>

editor.execute( 'bulletedList' )

Result:

<title></title>
<listItem listIndent="0" listType="bulleted">F{}oo</listItem>

[oskarwrobel]

We decided to go with the following model structure:

<title>
    <title-content>Title content</title-content>
</title>

It makes it possible to set Schema rules for all elements inside the title element. So, we can disallow all elements except title-content so all commands that could rename title to some other block should be disabled when selection is in the title element.