Closes #389

Grouping in the "list" command was completely broken from the start,
unlike "count group".  I have no idea what I have been thinking when I
wrote commit ae178c77, but it didn't
make any sense.

This commit is a rewrite of the feature.

For clients to be able to detect this feature, this commit also
increments the protocol version.

Keep MPD's footprint small until playback is requested.

Remove them as parameters from `db_selection_print()`.

Fixes `-Werror=odr`

Prepare to allow more complex expressions.

Will allow more complex fitler expression, such as negation (#89).

Modeled after std::filesystem::operator/() from C++17.

And specify whether generic tags are being scanned.

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

read_stream_art uses PRIu64 unconditionally with the Format
method of a Respone instance to output a size_t typed value.
If size_t is 32bit the output is garbeled. This patch uses
offset_type and PRIoffset to make sure the format string
and the type of the output value always match.

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

These attributes are printed in the "outputs" response, and the new
command "outputset" allows setting new values.

No attributes are currently implemented.

Closes #172