Header cleanup in Qt4ProjectManager.

Disentangle the covariant returns and introduce
convenience accessors.
Move internal classes away from Qt4Project.

Change-Id: I6c3158988824d2a159b1b3f8ecdf8432c32be2bc
Reviewed-on: http://codereview.qt.nokia.com/3216
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

src/plugins/qt4projectmanager/externaleditors.cpp

@@ -32,6 +32,7 @@
 
 #include "externaleditors.h"
 #include "qt4project.h"
+#include "qt4target.h"
 #include "qt4projectmanagerconstants.h"
 #include "qt4buildconfiguration.h"
 
@@ -142,7 +143,7 @@ bool ExternalQtEditor::getEditorLaunchData(const QString &fileName,
     // Get the binary either from the current Qt version of the project or Path
     if (const Qt4Project *project = qt4ProjectFor(fileName)) {
         if (const Qt4BaseTarget *target = project->activeTarget()) {
-            if (const Qt4BuildConfiguration *qt4bc = target->activeBuildConfiguration()) {
+            if (const Qt4BuildConfiguration *qt4bc = target->activeQt4BuildConfiguration()) {
                 if (const QtSupport::BaseQtVersion *qtVersion = qt4bc->qtVersion()) {
                     data->binary = (qtVersion->*commandAccessor)();
                     data->workingDirectory = project->projectDirectory();