The `SliceBuffer` constructor can throw.

This adds support for the full set of MPD 0.21 filter types.

That way, each plugin can decide to implement it better.

Apparently, Travis-CI will never upgrade its ancient Ubuntu Trusty,
so I have to get Boost from somewhere else.

Thanks to Martin Hierholzer for his PPA.

This passes the new `sort` and `window` attributes to the mounted
database.

Remove them as parameters from `db_selection_print()`.

May allow a better memory layout.

No undefined behavior.

Requires libmpdclient 2.15.