DSL: add accessibility_access stanza

[rolandwalker]

This stanza replaces the never-documented postflight methods enable_accessibility_access / disable_accessibility_access (remove_accessibility_access), and will also replace caveats / assistive_devices which is still present in the code for the moment.

The issue with the postflight approach is that the postflight blocks are opaque. We can't do anything very smart with the contents. In this case, that meant that Cask authors would need to build up and tear down accessibility permissions in each Cask:

postflight do
  enable_accessibility_access
end
uninstall_preflight
  disable_accessibility_access
end

This PR replaces the procedural logic above with a declaration:

accessibility_access true

Also:

• adds user feedback when accessibility is being changed
• fix trailing newlines on runtime-extracted bundle IDs
• de-document obsolete form assistive_devices
• create staged_test.rb for testing staged Casks
• test an actual staged Cask rather than mocking

Needed future work:

• accessibility_access should accept a bundle ID value for those cases where it cannot be deduced. Currently only the first app stanza is followed. accessibility_access can't be granted in Casks without an app stanza.
• the user should be able to turn off this feature.

@federicobond I might have introduced the newlines into bundle identifiers in #6329