Modified slide screencap process.

Instead of screencapping a slide right before it's shown, the client
will now screencap every slide before showing anything. This allows for
the duration timings to work properly.

Slide images are saved in the "queued" folder ("slide1.png",
"slide2.png", etc.). If that folder doesn't exist, create it before
running the code.

I have not been able to test this because I do not have access to a
Linux machine, so I branched it off in order to maintain stability. If
it works properly, it can be merged into the master.