New CTools Command Loading

With the introduction of this: #561788: ctools modal useage (...thats a bad title, it intros the reload command), a little usability thing came up.

When you use a modal for submitting the login form, after the form is submitted, you issue the reload command, the form remains displayed in the modal. So I set out to use ctools_modal_command_display() to display the loading message.

The problem is the loading message is all done via JS theme functions, not php theme functions. So I decided I needed a command.

It is trivial to write a command in my custom JS script, but I think this command would be useful for all. Attached is a patch