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.

xT/xT-highlighted.svg

@@ -0,0 +1,29 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg version="1.1" width="48" height="48" viewBox="0 0 48 48"
+   xmlns="http://www.w3.org/2000/svg">
+
+  <defs>
+    <radialGradient id="green-x">
+      <stop stop-color="hsl(66, 100%, 80%)" offset="0" />
+      <stop stop-color="hsl(66, 100%, 50%)" offset="1" />
+    </radialGradient>
+    <radialGradient id="orange">
+      <stop stop-color="hsl(36, 100%, 60%)" offset="0" />
+      <stop stop-color="hsl(23, 100%, 60%)" offset="1" />
+    </radialGradient>
+    <filter id="shadow" x="-50%" y="-50%" width="200%" height="200%">
+      <feDropShadow dx="0" dy="0" stdDeviation="0.05"
+         flood-color="rgba(0, 0, 0, .5)" />
+    </filter>
+  </defs>
+
+  <!-- XXX: librsvg screws up shadows on rotated objects. -->
+  <g filter="url(#shadow)" transform="translate(24 3) scale(16)">
+    <path fill="url(#orange)" stroke="hsl(36, 100%, 20%)" stroke-width="0.1"
+       d="M-.8 0 H.8 V.5 H.25 V2.625 H-.25 V.5 H-.8 Z" />
+  </g>
+  <g filter="url(#shadow)" transform="translate(24 28) rotate(-45) scale(16)">
+    <path fill="url(#green-x)" stroke="hsl(66, 100%, 20%)" stroke-width="0.1"
+       d="M-.25 -1 H.25 V-.25 H1 V.25 H.25 V1 H-.25 V.25 H-1 V-.25 H-.25 Z" />
+  </g>
+</svg>