Adding back selected-parent class

[jonwinton]

As components grow more complex, sometimes we need to write styles specific to edit mode. Sometimes in this situation we need awareness of when a component's parent is selected. There used to be a class for this, but in some updates it seems to have been removed. This adds that class back in.

Example
The in-article-slideshow component has a component-list of slides. If we want these all to be editable we need to show them all on the page, which makes the page very long and makes it so that the component cannot be dragged to different locations on the page. By having selected mode styles only we can maintain it's proper appearance and only expand it when we want to edit the component.

When not editing

When Editing

[coveralls]

Coverage decreased (-2.0%) to 72.851% when pulling 694d7b8 on parent-is-editing into 9ca5989 on master.

[dmabuada]

looks ok. +1