prepare sharing of UFOReader._getPlist implementation
use shared _getPlist implementation in UFOWriter
Repair and refactor UFOWriter._writePlist to be more DRY

`UFOWriter._writePlist` constructs the full path to the file to be written;
but the calling code did so, too.

This reliefs the calling code of `UFOWriter._writePlist` from knowing
how to construct the full path and thus makes the code more DRY.
Also, this fixes the faulty behavior that was caused by the double
path construction.
A further new helper `UFOWriter._deleteFile` was added to fully enable the
removal of path construction in the callers.
Add default layerOrder to writeLayerContents