Overview of changes in simple-scan 0.9.6

 * Show PDF files by default in save dialog
 * Set default save location to Documents directory
 * Remember save location
 * Attach JPEG files when emailing photos
 * Make stop button only sensitive when scanning
 * Correctly store default page orientation for first page
 * Optimise rendering
 * Show page throbber for pages following the first one
 * Fix bug where selecting scan before devices are detected would sometime fail
 * Don't move focus to selected page when moving the scrollbar
 * Make sure USB scanners are automatically selected when plugging in