Slide Timers

[carnage]

Ability to record time spent on each slide (eg during a rehearsal) and store this information.

Initial implementation should use a human readable text file.

As an extension it should be possible to reload a saved file and use it to compare times on a new run through. Perhaps a countdown instead of count up on the slide timers. Show in red if you go overtime.

See: dannyedel/dspdfviewer#34