Multi attach

When multiple files are selected and "Open with"->everpad is selected in nautilus, all of those files are now added as attachments to a single new note. The old behavior was to create a new note for each selected file and add that file as a single attachment.

Why is this useful? When I am scanning in paper documents, I produce one JPEG per page. The new behavior allows me to select all the images at once in the file manager and have them added to a single note, which I then title and tag. Then, when I finish, they are still selected so I can press one key to delete them and move on to the next document I scanned.

This is a very important feature to me and I will be using my development version until this feature is integrated into the ppa release.

I imagine this would be useful to others since people more often create a single note at a time rather than multiple notes each for a different single attachment.

I am not sure how to test the call to `pad.create_wit_attach`, so you'll need to do that before you accept this.