Editors: Automatically add editor/document id to context and use that

instead of manually adding a context everywhere.

Change-Id: I4336015a6d19349171a255e949da89b0013d6700
Reviewed-by: hjk <hjk121@nokiamail.com>

src/plugins/cppeditor/cppeditor.cpp

@@ -92,7 +92,6 @@ namespace Internal {
 
 CppEditor::CppEditor()
 {
-    m_context.add(Constants::C_CPPEDITOR);
     m_context.add(ProjectExplorer::Constants::LANG_CXX);
     m_context.add(TextEditor::Constants::C_TEXTEDITOR);
     setDuplicateSupported(true);