Clarify and clean up iterators and cursors for traversing the score

[cecilios]

Changes:

• Source files layouters/lomse_staffobjs_cursor.cpp and score/lomse_staffobjs_iterator.cpp moved to internal model folder
• DocIterator removed, as it was not used and was not needed. Files removed:
  • deleted: include/lomse_document_iterator.h
  • deleted: src/document/lomse_document_iterator.cpp
  • deleted: src/tests/lomse_test_document_iterator.cpp
    • Remove StaffObjsIterator, a useless and redundant wrapper for ColStaffObjsIterator:
  • deleted: include/lomse_score_iterator.h
  • deleted: src/internal_model/lomse_score_iterator.cpp
  • renamed: src/tests/lomse_test_score_iterator.cpp -> src/tests/lomse_test_staffobjs_iterator.cpp