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

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

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

No undefined behavior.

Requires libmpdclient 2.15.

Prepare to allow more complex expressions.

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

This enforces the "not nullptr" rule.

Prepare to make LightSong non-assignable.

Closes #284

Closes #283

Closes #258

Fixes "search already in progress" errors.

Fixes search within mount points, resulting in error "No such
directory".

Closes #190

No longer a problem with NDK r16.

The duration of a song can have fractions of seconds
(quote from http://www.upnp.org/schemas/av/didl-lite-v2.xsd):

    The format of the duration string is:
    H+:MM:SS[.F+], or H+:MM:SS[.F0/F1]
    Where:
    +H              one or more digits to indicate elapsed hours,
    MM              exactly 2 digits to indicate minutes (00 to 59),
    SS              exactly 2 digits to indicate seconds (00 to 59),
    F+              any number of digits (including no digits) to indicate fractions of seconds,
    F0/F1   a fraction, with F0 and F1 at least one digit long,
                    and F0 < F1.
    The string may be preceded by an optional + or - sign, and the
    decimal point itself may be omitted if there are no fractional  seconds digits.

Until now, a duration with fractions of seconds could not be parsed and
resulted in an unknown duration. Only durations in the format "H+:MM:SS"
were feasible. This commit enables to read durations in the first format,
i.e. "H+:MM:SS[.F+]"

If a directory is a mount point, omit the "directory: " as well.

This bug is years old, but has become more visible now that mount
points are persistent in the state file.

If `SimpleDatabase::Visit` is called on a database that contains a mounted directry the URIs of the elements passed to the callbacks are not prefixed by the mountpoint path. This leads to lsinfo and add not working because they use the wrong URI. This pull request is using the `WalkMount` helper function to create prefixed versions of `VisitDirectory`, `VisitSong` and `VisitPlaylist` to add the correct prefix to the parameters of the callback functions.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169