flatpak (build_manifest): some fixes needed after runtime update: - removed `--optimize=1` for lxml, as it is making the build fail - added build flag on x86_64 for Pillow (will need to be adapted if we build on other architectures) - added a module for Python 2.7.16, Python 2 not being included anymore in the runtime - DEFAULT_MANIFEST modules are put at the beginning of modules

These icons follow the Tango project [1] guidelines and have been downloaded from openclipart.org [2]

Free to use.

[1] http://tango-project.org
[2] https://openclipart.org