feat(ui): render the imagery split dividers from a store

createImagerySplit keeps the divider's value, drag, keyboard, CSS position
and scene split, and publishes its line to the imagery split store.
ImagerySplitView, in the scene chrome, teleports each line to the body, so
recent imagery and the Bhote Koshi event no longer build divider DOM.

docs/APPLICATION.md

@@ -206,7 +206,9 @@ screen (`#loading-screen` with a `.loader-status` line) as static markup so it
 paints before any script runs. `createApplication()` renders the views before
 anything reads the page; `omitViews` names views the application leaves out,
 such as `provider-settings`, and their hosts are removed. Leaving out
-`layer-panels` also leaves out the scene sharing dialogs it draws. An unknown
+`layer-panels` also leaves out the scene sharing dialogs it draws, and leaving
+out `scene-chrome` the before/after imagery dividers: the imagery still splits,
+with no line or handle to move it. An unknown
 view name is an error. A host that does not initialize Core's locale passes
 `localePicker: false` to `createApplicationControls` instead of editing the
 markup. Node tests that import application modules register the component