[Widgets] Support multiple nested editables of the same name

[Piotrek Koszuliński]

It would be useful if it was possible to initialise many nested editables using just one name:

editables: {
    label: {
        selector: '.someClass'
    },
},

More details about the current behaviour and reasons: ​http://ckeditor.com/forums/Support/Editable-fields-in-widget#comment-134623

Workaround - using ​widget.initEditable with dynamically generated names and selectors.

[Piotrek Koszuliński]

Motivated by ​PR#195 I started thinking how this feature could be now added to the editor.

First thing to notice is that any changes here cannot blow up existing implementations. Therefore, this feature should be opt-in:

editables: {
    label: {
        selector: '.someClass',
        // Enables multiple editables mode.
        multi: true
    },
},

This will make sure that existing implementations are safe.

The next thing is how editables should be stored in widget.editables. I think that the most natural way is that in an array. However, we must remember that we must not break existing implementations. So singular editables should not be wrapped in an array.

Finally, how to find an instance of editable from DOM. I think that the way proposed in ​PR#195 is ok, so in case of a multi-editable we need additional attribute. I would call it data-cke-editable-index, because proposed "id" conflicts with existing concept of editable names.

These changes mean that we need a new methods and changes in existing ones:

• new method - widget.getEditable( CKEDITOR.dom.element ) - because the logic got more complicated. This method should be now used everywhere - setFocusedEditable, setupWidgetLifecycleEnd, etc.
• new method - initMultiEditables() - to not pack too much logic into initEditable(). initMultiEditable() should reuse initEditable(). The decision which method to use should be made in setupEditables().
• I would do the same with destroyEditable(). Lets add destroyMultiEditable().