OpenH264 encoder element properties

These properties help to improve performance when really fast encoding is needed:
* deblocking - allows to disable deblocking entirely or only do deblocking within slices but not slice boundaries
* background-detection, adaptive-quantization, scene-change-detection are fairly self explanatory and can improve performance at the cost of quality
* slice-mode - I have exposed only two of what I considered to be the most clear and useful options from the openh264 library: fixed number of slices (specified in the num-slices property below) or automatic number of slices equal to the number of threads
* num-slices - when using slice-mode=n-slices, this specifies the number of slices to use

Finally, in a separate commit **I changed the default to slice-mode=auto** which combined with threads=0 makes OpenH264 automatically figure out how many threads and slices it will use.

**Note: The OpenH264 library current limits the maximum number of threads to 4** - https://github.com/cisco/openh264/issues/1489
Is there no enum in openh264 that we could reuse here instead of magic numbers?