Improve the detection abilities of our Flash auto-activation feature

As mlk pointed out in our 1.0 release reddit thread, our detection method for Flash is somewhat hackish:
http://www.reddit.com/r/linux/comments/9wake/caffeine_for_linux_10_released_now_able_to/c0estun

Since the Flash auto-activation feature seems to be important to people, I would really like it if we could find some way of improving it to give us a better guarantee that it will work for Flash videos of any size.

The simplest thing that I can think of would be to change the auto-activation routine to keep Caffeine active whenever there is a Flash##### file in the /tmp directory. This would be hackish in a different way, since the file stays there after the video is finished playing, until the user closes the tab that the video is embedded in. Another option would be to wait until the FLV video file is finished buffering (using our current method) and then determining how long it would play for if run begginning to end. If it is a 13 minute and 23 second flash video, then we could auto-activate for that period of time. There would still be the possible problem of the user pausing the video, but I would feel that failure would be somewhat less likely, so it could be a worthwhile change.

The best option, of course, would be figuring out some method for determining if a video is actually playing. This would, of course, be the most difficult option.