(WIP) Add help as valid kwarg to argument, and add panel concept to parameter decorators

[dwreeves]

• help= is now a valid kwarg for @argument() decorator.
• show_arguments is now Optional[bool]. If None, then the Arguments panel shows if (1) at least one help text is set for an argument, and (2) group_arguments_options is False.
• (Still WIP) panel= is now a valid kwarg for all parameter decorators. Panel specifies which panel (in <1.9, a "group") a parameter should be associated with.
  • (Still WIP: silently deprecating "groups", and renaming this to "panels". We will need to handle this with care, as we do not under any circumstances want to break existing 1.8.x code.)

Also WIP: tests, tests, tests.