Use simple login dialog to gain permissions on stack opening

If a user doesn't have enough permissions to view a stack, a simple login dialog
is now shown. It allows the user to (re-)login and automatically initializes the
page with the current URL (again).
Tracing tool: forget current stack when destroyed

Setting the stack to null makes sure a tracing tool objects gets
properly initialized when register() is called after destroy() was
called on the same object.
Init from URL: don't try to open same stack multiple times
Merge branch 'login-dialog'