ExpanderRules: Coding style update

Also makes the ExpanderRule getters const. No other functional change.
Expander: also read rules from data directories

* ExpanderSettings: Use a settings directory "expander" and rename
  settings file to "settings".
* ExpanderRules: Read expander rules from "rules" files in the settings
  directory, then from files in expander/rules subdirectories of the
  installation location data directories, and finally add the built-in
  rules. This allows packages to provide expander rules (as already
  done by the p7zip package).
* OptionalPackages: Remove AddExpanderRuleToHaikuImage invocations.
* ImageRules: Remove Expander rule file related rules.