Redo layer creation to create a sub layer if needed

[madsmtm]

Redo layer creation to match what's outlined in gfx-rs/wgpu#6107.

A deviation from that PR is that we track the bounds and scale factor from the super layer using observers. This fixes the issue in gfx-rs/wgpu#6107 (comment), and is also what I intend to do in Wgpu, but it's just a huge hazzle to do correctly without objc2, which Wgpu doesn't yet use.

The new API is:

• Layer::from_layer
• Layer::from_ns_view (replaces appkit::metal_layer_from_ns_view and appkit::metal_layer_from_handle)
• Layer::from_ui_view (replaces uikit::metal_layer_from_ui_view and uikit::metal_layer_from_handle)

Fixes #11 by removing dependency on raw-window-handle (since it's one line for the user to get a layer from the handle).

I have tested this with a fork of ash, of vulkano and of wgpu, seems to work fine there.

[madsmtm]

I think I intend to release v1.0 fairly shortly after this, as I feel like the API surface is now quite small, and I've spent a long time ensuring that the behaviour is as expected.