ProjectExplorer: Create initial project tree

Create an initial project tree with a ProjectNode and a FileNode for
the project file itself.

Fix the Projects to not implement their own tree before they have
better data.

Change-Id: I147ccd5603d22d1d60880a97f30fd8c271eac88c
Reviewed-by: hjk <hjk@qt.io>

src/plugins/genericprojectmanager/genericproject.h

@@ -94,7 +94,6 @@ private:
     QString m_filesFileName;
     QString m_includesFileName;
     QString m_configFileName;
-    QString m_projectName;
     GenericProjectFile *m_filesIDocument;
     GenericProjectFile *m_includesIDocument;
     GenericProjectFile *m_configIDocument;