set sensitivity of "save" independently of "view source"

[gue5t gue5t]

Right now, the check for whether a page can be saved is based on whether the page's source can be viewed. This patch changes that behavior to see whether a webkitwebresponse exists for the page, allowing images in their own tabs to be saved without redownloading. This only applies to the main action (ctrl+S or file->save [as]); right-clicking on images is unchanged.

[Cris Dywan (kalikiana)]

I like this. We should remove the comments whilst addressing this. And, this needs a check for view->mime_type != NULL so that you don't see Save As on frozen pages. Committing a change based on the proposal. Thanks!