ENH: Added option for setting the layoutManager in the main application

It is useful for slicelets, where the main window is not created and this the slicer.app.layoutManager() is not set.

Example for setting up a layout manager in a slicelet:

layoutWidget = slicer.qMRMLLayoutWidget()
layoutManager = slicer.qSlicerLayoutManager()
layoutManager.setMRMLScene(slicer.mrmlScene)
layoutManager.setScriptedDisplayableManagerDirectory(slicer.app.slicerHome + "/bin/Python/mrmlDisplayableManager")
layoutWidget.setLayoutManager(layoutManager)
slicer.app.setLayoutManager(layoutManager)
layoutWidget.setLayout(slicer.vtkMRMLLayoutNode.SlicerLayoutFourUpView)

somelayout.addWidget(layoutWidget)

From: Andras Lasso <lasso@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24132 3bd1e089-480b-0410-8dfb-8563597acbee