Detect print requests accross all browsers

This piece of code reliably detects a print request in:
- Firefox 5+ (not needed in 18+ because of mozPrintCallback)
- Chrome 9+
- Safari 5.1+
- Internet Explorer 9+
- Opera 15

What remains is to implement "TODO: PDFView.renderAllPages();".
(yury suggested to skip image decoding and text layer generation)