Add a Fyne frontend for xC

It is fairly mediocre all around, but also generally usable,
natively covering mobile platforms.

README.adoc

@@ -142,6 +142,12 @@ For remote use, it's recommended to put 'xP' behind a reverse proxy, with TLS,
 and some form of HTTP authentication.  Pass the external URL of the WebSocket
 endpoint as the third command line argument in this case.
 
+xA
+~~
+The Fyne frontend supports all of Linux, FreeBSD, Windows, macOS, Android, and
+iOS natively, albeit somewhat poorly.  Only use `fyne` or `fyne-cross` after
+running `make` first.
+
 xW
 ~~
 The Win32 frontend is a separate CMake subproject that should be compiled