Merge remote-tracking branch 'ivan/improve-zip-container' into morevna

* ivan/improve-zip-container:
  Change extension of zip-container to "sfg"
  Fix bug with zip-container
  Remove debug code
  Allow to "export" external canvas
  Split large macro definition for better debugging
  fix warning in filecontainerzip.cpp
  Derive FileSystem::(Read/Write)Stream from std::(i/o)stream

Conflicts:
	synfig-core/src/synfig/filecontainerzip.cpp