Add .wp-gallery to the placeholder image to the default placeholder, which is shown if the gallery doesn't have any images.
More adjustments to how selecting a gallery block is handled.

* Don't deselect when clicking on the HTML node. This means that the block is still selected when clicking on the scrollbar.

* When replacing a placeholder, if a gallery has been selected, reselect it. This fixes an issue where the selection range is adjusted causing the normal browser selection to display.

* Don't allow click events on a gallery block to propagate.
Don't add div/span to the set of valid elements for now.
Adjust selected class to use a namespace, tweak the gallery block styles, and add support for handling of backspace/delete so that it works properly in IE.

In order for the gallery block to be deleted when hitting backspace, we need to detect when the previous node is a gallery block and handle it manually.