animate: allow 3rd party plugins to provide additional animations

[ammen99]

The API for it is installed in the wayfire/plugins/animate/animate.hpp header.

How it works:

1. The external plugin providing extra animations needs to implement the actual animation by deriving from (and implementing the methods of) the animation_base_t class:

class animation_base_t
{
    virtual void init(wayfire_view view, wf::animation_description_t duration, animation_type type)
    {}

    /**
     * @return True if the animation should continue for at least one more frame.
     */
    virtual bool step()
    {
        return false;
    }

    /**
     * Reverse the animation direction (hiding -> showing, showing -> hiding)
     */
    virtual void reverse()
    {}
};

2. The plugin can register its animation type with the animate plugin by using the animation effects registry class:

// Automatically fetches the global instance of the effects registry
wf::shared_data::ref_ptr_t<wf::animate::animate_effects_registry_t> effects_registry;

effects_registry->register_effect("coolneweffect", wf::animate::effect_description_t{
    .generator = [] { return std::make_unique<cool_new_effect_t>(); },
    .default_duration = [cfg_option] { return cfg_option->get_value(); },
});

// On unloading the plugin:
effects_registry->unregister_effect("coolneweffect");