Tweak to allow minimum size for proceed popup

In versions of calibre prior to 2.7 it was possible for the user to increase the size of the job completion notification (proceed_question) dialog. This was useful for users of my Overdrive Link plugin, which can return a large detailed message. Now the notification popup is a small fixed size, which is best for most users, but those (like myself) with large screens would benefit from it being larger.

I am submitting for your consideration the implementation of a tweak to allow the minimum dimensions of the popup to be set.

This is just a suggestion. Perhaps you may have a better way to accomplish this.