avogadro-1.100.0-bp160.1.2

- Update to version 1.100.0:
  * Long list of changes, see
    <https://github.com/OpenChemistry/avogadroapp/releases/tag/1.100.0>
- Switch to Qt6 for build.
- Set up for tests, but these cannot be built as of now due to
  dependency on paraview's QtTesting (unavailable for openSUSE).
- Disable Avogadro_ENABLE_RPC to avoid depending on Molequeue,
  which is abandoned and does not build with Qt6.
- Fix builds for 15.6 by requiring gcc-c++ >= 8 for <filesystem>
  support.