VisualBuilder: Hook up everything needed for widget property editing.

It's now possible to edit widget properties inline in the properties window.
We're currently relying on the basic GVariant conversion functions to do
all the "parsing" but that's not gonna be good enough.

LibGUI/GVariant.cpp

@@ -101,6 +101,17 @@ GVariant& GVariant::operator=(const GVariant& other)
     return *this;
 }
 
+GVariant& GVariant::operator=(GVariant&& other)
+{
+    if (&other == this)
+        return *this;
+    // FIXME: Move, not copy!
+    clear();
+    copy_from(other);
+    other.clear();
+    return *this;
+}
+
 GVariant::GVariant(const GVariant& other)
 {
     copy_from(other);