Error message sent to client was "basic_string::_M_construct null not
valid" due to passing nullptr to the std::string constructor.

Regression caused by commit 386688b8

release v0.20.16

Closes #173

Fixes deadlock bug.  Closes #210

The PulseOutput needs to be "enabled" before WaitConnection() may be
called.

Closes #207

This commit also puts an instance of RemoteTagScanner into the
Instance class, and hooks it into the "add" and "addid" commands.

Fixup for commit 681e012f

Works around a deadlock bug which happens when waiting for an
EventLoop which isn't running anymore.

Fixes another part of #208, the one which caused the bogus exception.

Fixes deadlock due to mutex double lock.

Closes #208.

Allows more forward declarations.

To be used when a URL gets added.