src/QGst/message.* rename DurationMessage to DurationChangedMessage

Actually modified the following files. Its all just propagating the
message rename (Done to match the name of the 1.0 message.)
  modified:   src/QGst/enums.h
  modified:   src/QGst/global.h
  modified:   src/QGst/message.cpp
  modified:   src/QGst/message.h
  modified:   tests/auto/messagetest.cpp