Merge branch 'tiagogomes/includes-1.12-backport' into 'bst-1.2'

buildstream/_elementfactory.py

buildstream/_exceptions.py

buildstream/_frontend/app.py

buildstream/_frontend/widget.py

buildstream/_includes.py

buildstream/_loader/loader.py

buildstream/_loader/metaelement.py

buildstream/_loader/metasource.py

buildstream/_options/optionpool.py

buildstream/_pipeline.py

buildstream/_platform/linux.py

buildstream/_platform/platform.py

buildstream/_platform/unix.py

buildstream/_plugincontext.py

buildstream/_project.py

buildstream/_sourcefactory.py

buildstream/_stream.py

buildstream/_versions.py

buildstream/_yaml.py

buildstream/element.py