Use universal helper for writing toplevels to QDebug streams

Toplevel::debug() is one of annoyances that you need to deal with when
implementing a new client type. It can be tempting to just write "this"
to the stream, but it will result in a crash.

In order to make implementing new client types easier, this change
introduces a debug stream insertion operator overload that works for all
kinds of the Toplevel class.