Minor adjustments to classes

Renames conf/Conf to config/Config across the add-on and makes the Trait
enum available from the classes.services package directly.
Adjustments to playback and shortcut hooks

For the "show question" and "show answer" hooks, these now use a proper
addHook() call rather than wrapping a private member.

_keyHandler continues to be wrapped, but has a few improvements, namely

- if the user sets a shortcut to a built-in shortcut, only AwesomeTTS
  will run now instead of both AwesomeTTS and the built-in shortcut,
  EXCEPT

- if the user sets a shortcut to be F5 or R, the built-in sound(s) will
  play followed by the AwesomeTTS sounds.