QMake: Remove unnecessary build dir changed

Previously every time a qmake project was built, the
"buildDirectoryChanged()" was emitted, leading to a complete
reparse of the project file, tests, qml files and qml imports.

To keep "QmakeBuildConfiguration::updateProblemLabel" be called,
we re-introduce the "buildDirectoryInitialized" signal.

Task-number: QTCREATORBUG-27785
Change-Id: I32d881eacb8cf26050c33da643c74c2cab4dc22d
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/projectexplorer/buildconfiguration.h

@@ -116,6 +116,7 @@ public:
 
 signals:
     void environmentChanged();
+    void buildDirectoryInitialized();
     void buildDirectoryChanged();
     void enabledChanged();
     void buildTypeChanged();