Track who is hovered, apart from what consumes

Hover was per-sensor state that only changed when the walk reached that
sensor, so ending it depended on the walk, which consumption cuts short.
`CursorSenses` now keeps the set of widgets the cursor was inside, in a
new `Event::Global` slot for state a whole event type owns rather than
each widget -- which is also where the input restructure keeps its pointer
capture.

The walk visits only widgets the cursor is inside and stops at the layer
that consumes, as before. Whoever was in the set and is not now has been
left or covered, and gets its `HoverEnd` afterwards, however early the
walk stopped.

Two things fall out. `SensorState` is gone: whether a hover is starting,
on or ending is the difference between the two sets. And the consumption
line loses its `&& in_shape`, since being inside is now the reason the
widget is looked at rather than something to test again.

Nine tests, five of which fail on `main`. `hover_starts_and_ends_once_each`
pins the lifecycle, and `covering_a_widget_ends_its_hover` now returns the
cursor so an uncovered widget hovers again.

core/src/event/manager.rs

@@ -79,6 +79,7 @@ type EventData<Rsc, E> = (E, Rc<dyn for<'a> EventFn<Rsc, <E as Event>::Data<'a>>
 pub struct TypeEventManager<Rsc: HasEvents, E: Event> {
     // TODO: reduce visiblity!!
     pub active: HashMap<LayerId, HashMap<WidgetId, E::State>>,
+    pub global: E::Global,
     map: HashMap<WidgetId, Vec<EventData<Rsc, E>>>,
 }
 
@@ -107,6 +108,7 @@ impl<Rsc: HasEvents, E: Event> Default for TypeEventManager<Rsc, E> {
     fn default() -> Self {
         Self {
             active: Default::default(),
+            global: Default::default(),
             map: Default::default(),
         }
     }

core/src/event/mod.rs

@@ -9,6 +9,9 @@ pub use rsc::*;
 pub trait Event: Sized + 'static + Clone {
     type Data<'a>: Clone = ();
     type State: Default = ();
+    /// State the whole event type keeps, as opposed to `State`, which each
+    /// widget keeps its own of.
+    type Global: Default = ();
     #[allow(unused_variables)]
     fn should_run<'a>(&self, data: &Self::Data<'a>) -> Option<Self::Data<'a>> {
         Some(data.clone())