Add support for the Print module

When using the print module it generates a page by running all the preprocess hooks and process hooks, including preprocess page.

It doesn't always have a proper node object in its variables though so you can end up with this:

Notice: Undefined property: stdClass::$type in edit_preprocess_page() (line 393 of /sites/all/modules/contrib/edit/edit.module).
Notice: Trying to get property of non-object in edit_preprocess_page() (line 394 of /sites/all/modules/contrib/edit/edit.module).

I would probably think that the edit module should not be doing anything in here for pages being printed so maybe something like:

<?php
if (empty($variables['print'])) {
  // Do edit stuff.
}
?>

I don't really love that solution because other modules could easily add a 'print' variable, however there isn't really a better way to determine when printing.

Possibly you could check for $variables['print']['content'] or something to be a bit more specific.

There is also a semi-related issue of the print module where it adds pseudo node objects to the $variables that don't have things like a node type, which is likely to cause issues with many modules, not just edit.

Not sure how you would want to handle this one.