Changes:
* Tune configurations
* Removed the deprecated/old methods
* Add/Remove (unused) comment
* Remove unessential methods
* Replace conditional chaining (?) with traditional ones
  to make it works on all GNOME versions
* Add shell version compatiblity on configKeys
* Add missing statements
* Minor UI fix
* Updated README, CONTRIBUTING,
  CHANGELOG, and .gitignore
* Updated installer script

See: OLDER_VERSIONS_NOTE.md or README.md for
more information about older GNOME Version
support removal