Fixed|libdeng2: IndirectRule's value becomes zero when unset

When the source rule of the indirect rule was removed, the indirect
rule did not invalidate itself. Instead, the last value before the
unsetting remained in effect.

doomsday/libdeng2/src/widgets/indirectrule.cpp

@@ -40,6 +40,8 @@ void IndirectRule::unsetSource()
 {
     independentOf(_source);
     _source = 0;
+
+    invalidate();
 }
 
 void IndirectRule::update()