lazy-load expensive beatmap attributes to show a UI sooner

[tybug]

what matters most to users is that a screen is shown as soon as possible after they click "visualize". This is an attempt to make that process faster by not loading a beatmap's hitobjects (and other attributes that rely on the hitobjects) in the initial rendering of the ui, and load them in a separate thread instead (the completion of which we wait for before actually visualizing anything, which was easy to do, since we already have that logic for the existing process_sliders thread).

What I have here does work with no issues, which is why I'm pring it. The thing is, this adds a fair bit of complexity to the code and I'm not sure the speedup is all that significant (though it's a damn hard thing to test). I think I'm not going to merge this for now, but may change my mind in the future.

Note to future self: loading the beatmap itself (not even its hitobjs) from slider is likely to be expensive and should also be given the same lazy-load treatment for best results. Specifically this code:

        if beatmap_info.path:
            self.beatmap = Beatmap.from_path(beatmap_info.path)
        elif beatmap_info.map_id:
            # TODO move temporary directory creation to slider probably, since
            # this logic is now duplicated here and in circlecore
            self.beatmap = self.library.lookup_by_id(beatmap_info.map_id, download=True, save=True)