Add a command to manually toggle thumbnail generation

[FichteFoll]

Via tomasklaen/uosc#445.

Thumbfast currently does not have a way to toggle the generation and/or display of thumbnails. It should provide a way to toggle them and to specify the default operation mode, i.e. whether it should start generating them on first load/request, much like there already is such a configuration option for network videos.

Additionally, this toggle should also work for network videos.

I could see a global toggle and a per-video toggle (when working with playlists).