Add Build to the right click menu of sub projects for Qt4 Projects

They do what you expect them to do, that is build only a subtree.
The implementation is a little bit strange. We temporarly set the sub
node to be built on the BuildConfiguration, enqueue that
BuildConfiguration (which calls BuildStep::init()), and then reset that
temporary value.

A more general way would be to have the ability to extend
BuildConfiguration::buildProject() in a way that additional data can be
passed to the BuildSteps.

Task-Nr: QTCREATOR-5
Task-Nr: QTCREATORBUG-44

src/plugins/qt4projectmanager/makestep.cpp

@@ -104,7 +104,11 @@ bool MakeStep::init()
     Environment environment = bc->environment();
     setEnvironment(environment);
 
-    QString workingDirectory = bc->buildDirectory();
+    QString workingDirectory;
+    if (bc->subNodeBuild())
+        workingDirectory = bc->subNodeBuild()->buildDir();
+    else
+        workingDirectory = bc->buildDirectory();
     setWorkingDirectory(workingDirectory);
 
     QString makeCmd = bc->makeCommand();