use strings to connect nodes

This makes the node system somewhat more high-level with the intent of making
working with them far more flexible and stable. By making the architecture more
abstracted, it becomes far less rigid which should allow us to do even more
with it and make it much less crash prone.

app/widget/keyframeview/keyframeview.cpp

@@ -31,14 +31,14 @@ KeyframeView::KeyframeView(QWidget *parent) :
   setAlignment(Qt::AlignLeft | Qt::AlignTop);
 }
 
-void KeyframeView::SetElementY(const NodeConnectable::InputConnection &c, int y)
+void KeyframeView::SetElementY(const NodeInput &c, int y)
 {
   qreal scene_y = mapToScene(mapFromGlobal(QPoint(0, y))).y();
 
   element_y_.insert(c, scene_y);
 
   for (auto it=item_map().cbegin(); it!=item_map().cend(); it++) {
-    if (it.key()->parent() == c.input && it.key()->element() == c.element) {
+    if (it.key()->key_track_ref().input() == c) {
       it.value()->SetOverrideY(scene_y);
     }
   }
@@ -60,7 +60,7 @@ void KeyframeView::SceneRectUpdateEvent(QRectF &rect)
 KeyframeViewItem* KeyframeView::AddKeyframe(NodeKeyframe* key)
 {
   KeyframeViewItem* item = super::AddKeyframe(key);
-  item->SetOverrideY(element_y_.value({key->parent(), key->element()}));
+  item->SetOverrideY(element_y_.value(key->key_track_ref().input()));
   return item;
 }