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+]"

This is not necessary, because a strlen() on a literal gets optimized
away by the compiler.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Eliminate one implicit std::string copy.

Reduce bloat.

Reduce bloat.

Expat can call CharacterData() multiple times if the CDATA contains
entity references.  We need to collect all of them in one large
string.

Reduce bloat.

Eliminate the std::string bloat.

Reduce runtime bloat.

Reduce overhead by omitting the part of the buffer that consists only
of whitespace.

Replaces "m_title" and "m_props".  More bloat removed.

Reduce bloat.

There is no use in duplicating these containers.  It only means that
we have to search both, duplicating the code.

Call titleToPathElt() only once for each object.

"upnp:class" is an element, not an attribute of element "item".  This
fixes a regression from commit 65ebfb16

Don't add to the std::map.

Don't store in the std::map.

Don't put all <res/> attributes to the attributes map; parse the
"duration" attribute as soon as we receive it, and store it in an
integer attribute.  This reduces bloat.