Allow admin theme CSS for dialogs to be used on front-end

[quicksketch]

In #1725, we encountered an issue where it would be desirable to load the admin theme dialog styling rather than the front-end dialog styling. Take this example of deleting a block from the front-end:

As this is an admin-action, it may make sense to have this be styled in the admin theme.

[quicksketch]

This would take some work, but I think it might be doable:

• In places where we want a front-end dialog to use the admin theme, add a special class to the dialog, such as dialog-admin-theme.
• Separate all admin theme CSS for dialogs into a separate file (this is already done for Seven).
• Change or duplicate all classes in the CSS file from .ui-dialog to .dialog-admin-theme.
• Add a line to the theme .info file for dialog_stylesheets[] = css/dialog.css (or something).
• If the special class dialog-admin-theme was placed on the dialog, add an AJAX command to load the special dialog CSS file from the current admin theme.

This approach is essentially the reverse of what we do for CKEditor, where the front-end theme is loaded into the editor contents, even though the rest of the page is using the admin theme. We also use a line in the .info file:

ckeditor_stylesheets[] = css/ckeditor-iframe.css

[indigoxela]

I wonder if this ever got implemented elsewhere...

In fact, the admin theme gets applied to dialogs under some circumstances and causes trouble. See #5250