Windows: Dont launch calibre as an external viewer

Windows: When viewing books with an external viewer, check if windows
will open the file with calibre itself, and if so, popup an error
message telling the user to install some software capable of viewing the
file. Fixes #1249698 [Opening a PDF in calibre changes the default "Open with" app](https://bugs.launchpad.net/calibre/+bug/1249698)