Let a host application place live contacts

createApplicationCatalog takes an optional placement provider. Its
layer(kind) hands Flights, Military Flights, AIS Vessels, Transit and Rail
a place(id, wallMs), and optionally observe(id, fix) and forget(id). While
live, each layer tells it every fix it accepts and draws a contact where it
places it: position, height (the layer's own when none is given), course
and speed. A null answer, a provider that throws, a kind it leaves out, or
no provider at all keeps the layer's own motion unchanged, and following
the world clock the layers never ask it. A transit vehicle or train it
holds still lets the render loop idle and is asked again each second;
vessels ask for frames only while it places one, and stop when the layer
turns off.

Tests show the layers draw and reconcile exactly as before without a
provider and with one that places nothing, draw where a provider places
them, and idle while it holds them still.

changelog.d/1737-placement-provider.md

@@ -0,0 +1,7 @@
+- Let a host application decide where live contacts are drawn.
+  `createApplicationCatalog({ placement })` takes a placement provider whose
+  `layer(kind)` hands Flights, Military Flights, AIS Vessels, Transit and Rail
+  a `place(id, wallMs)` (and optionally `observe(id, fix)`, `forget(id)`):
+  each layer tells it every live fix it accepts and draws a contact where it
+  places it. Without one, or for a contact it does not place, every layer
+  moves as before (samehkhamis, #1737).