ProjectExplorer: Make the FlatModel a Utils::TreeModel

The FlatModel is essentially a proxy model keeping expansion and
filter state per ProjectTree(View). Using a Utils::TreeModel makes
it fast enough to allow recreation of the proxy structure on
structural changes simplifying the parent/child logic significantly.

The {Session,Project,Folder,File}Node hierarchy still is still primary
information and shared by all views.

Change-Id: Ic08180a19bda37908280ff30e0737d188ed93e92
Reviewed-by: Robert Loehning <robert.loehning@qt.io>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/qmakeprojectmanager/qmakenodes.cpp

@@ -2083,15 +2083,7 @@ void QmakeProFileNode::applyEvaluate(EvalResult *evalResult)
         removeProjectNodes(projectNodes());
         removeFolderNodes(folderNodes());
 
-        bool changesShowInSimpleTree = showInSimpleTree() ^ showInSimpleTree(result->projectType);
-
-        if (changesShowInSimpleTree)
-            ProjectTree::instance()->emitAboutToChangeShowInSimpleTree(this);
-
         m_projectType = result->projectType;
-
-        if (changesShowInSimpleTree)
-            ProjectTree::instance()->emitShowInSimpleTreeChanged(this);
     }
 
     //
@@ -2232,6 +2224,7 @@ void QmakeProFileNode::applyEvaluate(EvalResult *evalResult)
     updateGeneratedFiles(buildDirectory);
 
     cleanupProFileReaders();
+    ProjectNode::emitTreeChanged();
 }
 
 void QmakeProFileNode::cleanupProFileReaders()