#i124875# use AVAsset's best guess duration if AVPlayerItem isn't ready yet

in AV Foundation the most accurate media asset duration is available when the
AVPlayerItem's status changes to AVPlayerItemStatusReadyToPlay. There already
is an observer for this, but the upper layers expect getDuration() to be
synchronous and to return within a reasonable time. Since returning an
approximation is sufficient using the AVAsset's duration as fallback is fine.
#i124896# remove obsoleted sal-module-internal custom STL allocator

The SAL module avoided the heavy dependency on the stlport4-libraries by using
a custom allocator for its internal STL containers. With stlport4 removed these
dependencies are gone and the SAL-internal custom allocator is obsoleted. Since
the custom allocator results in build problems with clang>=3.4 or xcode>=5.1 it
is time to remove it for good.