Fixed strict standards warning on delete_dialog

When deleting a draft, we get a strict standards warning (see attached screenshot).

![screen shot 2013-08-27 at 11 17 28 am](https://f.cloud.github.com/assets/349765/1036953/18d82be0-0f46-11e3-8c4b-e31874820820.png)

To fix the bug, I just made quickmail::delete_dialog() a static method.