feat: animation events

[Kneemund]

Adds the ability to (continuously) request AnimationFrame pen events to render animations that are not strictly tied to other pen events.

• Needed (in some form) for Laser tool? (feature request) #476 and its upcoming PR.
• An additional struct Animation is added to the engine and used to ensure that only one AnimationFrame is in flight at a time.

Things to Discuss

• We can either "pull" handle_pen_event out of the engine and into the canvas, which is how it's currently done, or use EngineTasks to make the engine call itself (currently commented). The former seems cleaner and likely faster, the latter allows us to get rid of the additional EventResult field and request animation frames from anywhere with access to the engine (i.e. also in the event handlers themselves).
• Use a struct instead of the tuple (EventPropagation, WidgetFlags, bool) in the handle_pen_event functions.
• Give glib::source::idle_add_local_once a different priority or use a different method. I just used what works on my machine and seems correct given my limited knowledge of the glib event loop.

EDIT: Perhaps we could turn request_animated_frame into a field of PenProgress::InProgress. That would reduce the flexibility though.