Stop cyclic shell color references

Resolve color references iteratively and use the caller's fallback when a reference repeats. Preserve normal palette roles, color parsing and gradient first stops.

Reported-by: Aleksej <aleksejhairov@yandex.ru>

shell/Commons/Color.qml

@@ -49,9 +49,15 @@ QtObject {
   }
 
   function flatColor(value, fallback) {
+    var seen = Object.create(null)
     var token = firstColorToken(value)
     var role = String(token || "").replace(/^\s+|\s+$/g, "").toLowerCase()
-    if (root.shellValues[role] && root.shellValues[role] !== token) return flatColor(root.shellValues[role], fallback)
+    while (root.shellValues[role] && root.shellValues[role] !== token) {
+      if (seen[role]) return fallback
+      seen[role] = true
+      token = firstColorToken(root.shellValues[role])
+      role = String(token || "").replace(/^\s+|\s+$/g, "").toLowerCase()
+    }
     if (role === "foreground" || role === "text") return root.foreground
     if (role === "accent") return root.accent
     if (role === "urgent") return root.urgent