Add a Qt Widgets frontend to xC

This is very much a work in progress, though functional.

Qt Widgets are basically non-working on Android,
though Qt Quick requires a radically different approach.

README.adoc

@@ -33,9 +33,9 @@ including link:xC.adoc#_key_bindings[keyboard shortcuts].
 
 image::xP.webp[align="center"]
 
-xA, xW, xM
-----------
-The native frontends for 'xC'.  Using them is not recommended.
+xA, xT, xW, xM
+--------------
+Other frontends for 'xC'.  Using them is not recommended.
 
 xD
 --