New setting for disabling syncing of enhanced-h264ify settings via Firefox sync

[Stebs]

Currently, enhanced-h264ify seems to sync its settings (which codecs are allowed etc.) via Firefox Sync to different devices.

If those devices have different capabilities (PC can accelerate h264 and VP9 via GPU, laptop can only accelerate h264) this creates the problem that one cant optimize the setting based on device capabilities.
-For the above example, either PC cant use the more efficient VP9 codec, or laptop must use software decoding for VP9.

To avoid this, a setting to disable syncing enhanced-h264ify settings (without having to disable sync of all add-on settings) would help, so every device can be specifically optimized.
(See also Issue #8 enable based on device)