ProjectNodes: Do not derive Project Nodes from QObject

That should save some memory per node, and since creator has a lot of nodes
(e.g. opening the LLVM project adds about 1 000 000 nodes) this should be
noticeable:-)

Calling update inside ProjectTree::currentNode() and rename it to
findCurrentNode() to make sure it is an still existing pointer.
Also, try to reduce the somehow more expensive currentNode() calls
and sprinkle some const around that usage.

Change-Id: I6a7c5db01a71d53d39544d3013cad557d5b96cdc
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/genericprojectmanager/genericproject.cpp

@@ -126,7 +126,7 @@ public:
 
     bool showInSimpleTree() const override { return true; }
 
-    bool supportsAction(ProjectAction action, Node *) const override
+    bool supportsAction(ProjectAction action, const Node *) const override
     {
         return action == AddNewFile
                 || action == AddExistingFile