Add zstack view to compose views on top of each other

[viktorstrate]

Adds a new zstack Xilem view (along with an accompanied Masonry widget).
The view is inspired by the ZStack view in SwiftUI, which composes the child view on top of each other.

This pull-request is still work-in-progress.

• It doesn't allow for changing the layout when BoxConstraints of the children are different.
  (top-left, top-right, bottom-left, bottom-right, center).
• I want to update http_cat to put copyright on top of image like seen below.

[Philipp-M]

See also #591, which goes a similar direction.

I think to make a zstack really useful we would need transforms (kurbo::Affine) on widgets in general and relevant logic for pointer events etc. See also here. I'm thinking about getting to that topic soon, as I think this is the main blocker for allowing fancy interactive vector graphics with vello and xilem.

I like the general direction, i.e. unifying a potential xilem_svg API using a zstack (instead of separate context such as in the web).

[DJMcNab]

I don't think that arbitrary transforms should block this landing, though. I don't really have the bandwidth to look at this too closely at the moment, unfortunately.

[Philipp-M]

To clarify, I didn't mean this to be a requirement, it's orthogonal to this change (and a lot more work...). I'll take a look at the PR when it's out of draft. On a skim it seems reasonable so far.