* Fix P2 bug described by Bill Wharrie in "Problem dragging clips between tracks with different sample rates" thread on audacity-quality. Made sure the clips get resampled to the destination WaveTrack rate, and marked changed.
* Comment out unused override of WaveTrack::MoveClipToTrack().
* Added an ANSWER-ME about what appears to be a useless declaration and assignment.
In WaveClip.cpp, remove wxASSERT that Michael was trying to convince me to do yesterday. In debugging this, found a case when it's not a bad call. Thanks, Michael! :-)
In TrackPanel.cpp, removed some cruft.
better handling of error conditions in all BuildFromXML methods, per comment:
// BuildFromXML methods should always return a BlockFile, not NULL,
// even if the result is flawed (e.g., refers to nonexistent file),
// as testing will be done in DirManager::ProjectFSCK().
