Bug 815083 - Replace gallery delete confirm() call by CSS based window

Patch for [#815083](https://bugzilla.mozilla.org/show_bug.cgi?id=815083). At the moment the normal `confirm()` dialog is used to get confirmation for deletion of photo's in the gallery. This patch replaces it with the standard HTML/CSS dialog, adds the right text to it and adds a danger class to the delete button.

In terms of localisation, the `Delete` term is used in a variety of places, do I need to really add it here as well for all locales or do we have a shared way for terms like this?