Align: a key object needs two or more selected objects

Selecting two objects, clicking one to make it the key object and then
Shift-clicking the other left the remaining object as the key. The
canvas still drew that object with the thick key-object outline. The
Align panel aligned the object to itself, so the object did not move.

The key is now cleared after any change that leaves no other selected
object outside it, such as a Shift-click, or hiding or deleting the
other object's row in the Layers panel. On a single selected object,
or on a group selected with only its own members, select.key sets no
key.

With one object selected and Align to Key Object chosen, the Align
panel, the Control bar and the Properties panel ran object.align with
to: "key", which fails with "no key object" when the selection has no
key. All three now align a single selected object to the artboard
whatever Align To is set to.

Docs: docs/target-app-parity.md.

crates/engine/src/cmd/select.rs

@@ -57,7 +57,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Set Key Object",
             [],
             None,
-            "{id?: a selected object (the Selection tool: a click on one object of a selection of several; on the key again: none)} the key object Align aligns to and Distribute Spacing spaces from; none clears it",
+            "{id?: a selected object, one of two or more (the Selection tool: a click on one object of a selection of several; on the key again: none)} the key object Align aligns to and Distribute Spacing spaces from; none clears it, as does a selection that holds only the key and objects inside it (one object, or a group key and its own members)",
             has_doc,
             key
         ),