replaced Pos Audio::_pos by audio_frame_t _frame.

also replaced start/endRecordPos with an XTick
eliminate all Pos-objects in FRAME mode
eliminate all usages of Pos::setType. removed marker lock!

This will remove the "marker locking" functionality, which
allows one to have a marker placed always on a certain frame
(not tick), regardless of the tempomap.
eliminated all use of the Pos::type()

This should be mainly elimination of code which
is dead, because the "type()==FRAMES" considion
is never true.
Pos and PosLen can now write subticks