Add class / id to slideshows and galleries, headline/button to slideshows

This patch for the Respond 3 dev branch adds separate dialogs for slideshow and gallery images.  Both allow the user to set the image container's class and id.  The slideshow dialog also adds support for headline, button, button href, and button target.  Here is an example dialog:

![image](https://cloud.githubusercontent.com/assets/6088477/4498100/b07229cc-4a75-11e4-9fdb-82272c347fa6.png)

The imagesDialog code is shared between the two, but the dialogs and render scripts for slideshows and galleries are kept separate for simplicity and readability, and because there is very little overlap between the two dialogs.

There is one change needed to themes to make them compatible with the new slideshows.  Bootstrap's default positioning for .carousel-caption has to be overriden to move it to the top of the slide, like this:

.carousel-caption {
	bottom: inherit;
	top: 20px;
}

I've already made the above change in the simple and advanced themes included with Respond.

Please let me know if you have any questions or comments.