Smart Backgrounds support

[gjh42]

This is a bit annoying. I threw the not-yet-tested 3.0 zip up in another thread with a request to let me know about any problems. 33 people downloaded it, but not one came back with a question.

Moving the code from tpl_main_page to a function broke it, as $current_page_base is not available inside a function without special arrangements. $_GET['main_page'] is available everywhere, so I have switched the function to use that, and it now works correctly. I have also improved the readme with a note about the base image for foreground image switching.