The previous CAST_MOD(Impl) for the action_router constructor was
illegal because `this` is not yet of type Impl (because the subclass
constructor has not run yet).
The action_router now accepts std::function for its callbacks.
Fixes#2519
* module: Implement proof of concept action router
Action implementation inside module becomes much cleaner because each
module just registers action names together with a callback (pointer to
member function) and the action router does the rest.
* Make input function final
This forces all modules to use the action router
* modules: Catch exceptions in action handlers
* Use action router for all modules
* Use action_ prefix for function names
The mpd module's 'stop' action overwrote the base module's stop function
which caused difficult to debug behavior.
To prevent this in the future we now prefix each function that is
responsible for an action with 'action_'
* Cleanup
* actions: Throw exception when re-registering action
Action names are unique inside modules. Unfortunately there is no way to
ensure this statically, the next best thing is to crash the module and
let the user know that this is a bug.
* Formatting
* actions: Ignore data for actions without data
This is the same behavior as before.
* action_router: Write tests
