Bump to version 0.21.2

* Add support for reset all controls
* Add support for showing detected v4l controls
* Add sliders to adjust threshold filter level
* Make sliders reflect min/max/step values
* Use the newer V4L2 control functions
* Update po files and exclude the gtk2 data file from it
* drop gtk2 and legacy gtk3 support
* Embed mutexes inside struct camera
* Update README.md to reflect latest changes