Add a Fyne frontend for xC

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

xA/xA-highlighted.svg

@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg version="1.1" width="512" height="512" viewBox="0 0 512 512"
+   xmlns="http://www.w3.org/2000/svg">
+
+  <defs>
+    <linearGradient id="background" x1="0" y1="0" x2="0" y2="1">
+      <stop stop-color="#ffaa00" offset="0" />
+      <stop stop-color="#ffffff" offset="1" />
+    </linearGradient>
+    <clipPath id="clip">
+      <rect x="0" y="0" width="1" height="1" />
+    </clipPath>
+  </defs>
+
+  <rect x="0" y="0" width="512" height="512" fill="url(#background)" />
+
+  <g transform="translate(64, 64) scale(384)" stroke-linecap="square">
+    <g clip-path="url(#clip)">
+      <path stroke="#ff0000" stroke-width="0.125"
+         d="M 0.25,1.1 0.65,-0.1 M 0.75,1.1 0.35,-0.1 M 0.225,0.75 0.775,0.75" />
+    </g>
+  </g>
+</svg>