Navigation: choose the orbit centre: view centre, origin or point under the cursor (#1377)

Preferences ▸ Navigation ▸ Orbit around now offers View centre (the default, as before), Origin and Point under the cursor. A saved orbit_cursor preference migrates to the cursor option, and the control channel still accepts the old key.

Thanks to @kuraneko1 for this.

Co-authored-by: kuraneko1 <98832587+kuraneko1@users.noreply.github.com>

crates/ui-egui/src/control.rs

@@ -31,7 +31,7 @@
 //! - `ui.confirm {accept?}`: the Delete confirmation (without `accept`: what it lists)
 //! - `ui.shortcut {command, key?, replace?}`: a command's key, or bind one ("" clears)
 //! - `ui.help {item?: about|shortcuts|report}`: the Help menu; returns the version and diagnostics
-//! - `ui.prefs {...}`: the preferences (default_units, nav, zoom_reverse, orbit_cursor, msaa,
+//! - `ui.prefs {...}`: the preferences (default_units, nav, zoom_reverse, orbit_around, msaa,
 //!   length_decimals, angle_decimals); returns them all
 //! - `ui.capture {action?: capture|revert|cancel}`: the Capture Position question
 //! - `ui.agent_cursor {show?, speed?: 1–5 (times) | "instant", follow_camera?}`: the agent cursor