The attached patch adds support for Quansheng UV-R50 radios as requested in #6129. I've added support for all documented options.

I hit a small issue in TestCopyAll as one of the test cases involves a channel with a name containing spaces, and this radio doesn't know how to encode a space in a channel name as far as I can tell, so I added support in that test to ignore this narrow class of failures. I'm happy to change for another approach if it is preferred.

As others on the list have noted, I had to rm tests/images/Puxing_PX-777.img to make tests pass, but that failure seems unrelated to my changes. Without that file, all tox tests pass.

Per guidelines I'm attaching Quansheng_UV-R50.img at https://chirp.danplanet.com/issues/6129

Full commit message:

"""

[uvr50] New model: Quansheng UV-R50 (fixes #6129)

* Adds support for Quansheng UV-R50 radios.
* This may or may not support all UV-R50 radios, I only have one model
  to try it with and information on variants is scarce.
* This supports all options documented in the manual.
* Fix TestCopyAll to not fail radios that don't support certain
  characters in channel names, since this one doesn't do spaces.
--