Created QmitkXnatTreeModel. Added icons to xnatTreeBrowserView for dataModel, project, subject, experiment.
Conflicts:
Modules/XNAT/files.cmake
Plugins/org.mitk.gui.qt.xnat/src/internal/QmitkXnatTreeBrowserView.cpp
Plugins/org.mitk.gui.qt.xnat/src/internal/QmitkXnatTreeBrowserView.h