nitpicker: Parent-child relation between views

Fixes #1020
nitpicker: Respond to session-local focus change

This patch improves the focus handling by updating the menubar each time
the user clicks on a different view, even the old and new view belong
to the same session.
os/reporter.h: Don't enable if connection fails

If opening a report session fails (for example because of invalid
session arguments) and an exception is thrown during the session
construction, the report would wrongly keep the enabled state.
By moving the assignment of the state after the construction,
the report stays disabled as it should be.