[Masonry] Implement unified PointerEvent

[xorgy]

There is some consensus that a PointerEvent-style API (like existed in Glazier) is probably a good way to handle touch and mouse in general. This is a tracking issue for that topic.

[raphlinus]

There are a few layers to this. The simplest layer is just to enable mouse-like behavior from touch, so that touch can operate widgets like buttons and sliders, and also selection (and focus) in text input.

Another more sophisticated layer is to support multi-touch well. That has potentially far-reaching changes, as multiple widgets can be active at the same time. A careful design is needed.

And yet another more sophisticated layer is gesture disambiguation and recognition. A touch-drag gesture on a slider widget should move the slider, but on mobile, the same gesture not on a suitably sensitive widget should be interpreted as a scroll. There may be platform-specific gesture recognition code we want to call into, or it may be that we want a pure Rust cross-platform implementation of all this, tunable to feel native on the specific platform.

There may also be improvements desired on the winit side. I see a bunch of PRs to add pen support (rust-windowing/winit#2647 is the most recent) but they seem to be stuck in review. rust-windowing/winit#99 seems to be the main tracking issue there.

To be clear, I think we should have the simplest layer first, so we can unblock simple UI interactions on mobile, then carefully design what we want to do for the other two.