Add optional id/signature to Views generated queries

When troubleshooting performance issues on large sites with lots of views, spotting which queries were generated by Views (and even worse, which specific view is responsible) can be a hassle. The following patch adds an option in the admin/build/views/tools page to add a bit of 'signature' text to each view. When it's activated, a pseudo-field called 'view_name' will be added to each view, containing the name of the current view and the display style that's being used to generate its query. Super-handy, and easy to flip on and off during debugging.