Core: Clean up context

Move the context code into its own implementation file (from id.cpp)

Change-Id: I5e487e0ab0a9b3a33ef32f2d5b7a5d703bded099
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>

src/plugins/coreplugin/coreplugin.pro

@@ -24,6 +24,7 @@ SOURCES += mainwindow.cpp \
     fancytabwidget.cpp \
     generalsettings.cpp \
     id.cpp \
+    icontext.cpp \
     messagemanager.cpp \
     messageoutputwindow.cpp \
     outputpane.cpp \