Wizards: Open a relevant file in editor after the wizard run

Remove all hacks/conventions of considering the last generated
file as project file, etc. and instead add attributes flags to
Core::GeneratedFile, giving fine-grained control of what to do
with the file. Implement static utility functions in wizards
that handle it. Add boolean XML-attributes "openeditor"/"openproject"
to the file elements used by the CustomWizard XML-specification.
Manually set the attributes in all wizards.

Task-number: QTCREATORBUG-1166

src/plugins/texteditor/textfilewizard.cpp

@@ -54,5 +54,6 @@ Core::GeneratedFiles
     const QString fileName = Core::BaseFileWizard::buildFileName(path, name, suffix);
     Core::GeneratedFile file(fileName);
     file.setEditorId(m_editorId);
+    file.setAttributes(Core::GeneratedFile::OpenEditorAttribute);
     return Core::GeneratedFiles() << file;
 }