Introduce config.initialData

[pjasiun]

Right now, in all editors, you can use element or data as the first parameter. The problem is, that, in many cases, you want to use the element for your existing DOM, but use data from the database. The problem is even bigger when you use for instance React integration where we have no good way to inject HTML data to DOM.

Because if it many integrations uses:

InlineEditor.create( element, {
    // ...
} ).then( editor => {
    editor.setData( '<p>foo</p>' );
} );

but this is wrong, because:

1. it will create an undo step during the initialization,
2. it will break the collaboration if the collaboration plugin is used because you need to have initial data available in the collaborative editor during the editor initialization (this is the moment when the editor decides if local or server data should be used).

This is why we decided that initialData configuration should be introduced.

InlineEditor.create( element, {
    initialData: '<p>foo</p>'
} );

initialData will be used instead of elements HTML.

InlineEditor.create( '<p>bar</p>', {
    initialData: '<p>foo</p>'
} );

Exception will be thrown, such configuration does not make sense.

In the future we plan to move also source element to the configuration:

InlineEditor.create( {
    sourceElement: element,
    initialData: '<p>foo</p>'
} );

But since this is a bigger (and breaking) change, we agreed to pospond it and do it later.

[pjasiun]

Make sure to carefully update documentation of all editor.create methods to keep the documentation consistent.