Brushes: dragging a brush reorders it within its type

The reference app reorders a brush that is dragged to a new place in
its Brushes panel, among the brushes of its type only. VectorCraft's
Brushes panel could not reorder brushes, and no command moved one.

brush.move {name | names, to?, type?} moves brushes to index to among
the brushes of their type, in brush.list order, as one undo step. A
move that keeps the order records no step. Brushes of different types,
or a type other than theirs, are refused with "brushes move only within
their type". In the Brushes panel, a brush dragged onto another brush
of its type goes before or after it by the half of the tile or row
under the pointer, and a bar marks the slot while it is held there. A
brush dropped on a brush of another type stays where it is, and the
status bar shows the refusal.

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

docs/mcp.md

@@ -446,6 +446,11 @@ in the app runs `paint.setFill` (or `paint.setStroke`, the active proxy) with `i
 The Swatches panel menu's commands: `swatch.addUsedColors {selection?, global?}`, `swatch.unused` (a query: the
 names Select All Unused selects), `swatch.merge {names}` (the first is kept), `swatch.ungroup {name}` and
 `swatch.sortByKind`.
+`brush.move {name | names, to?, type?}` reorders brushes among the brushes of their type, as dragging one onto
+another of its type in the Brushes panel does. `to` is an index among the brushes of that type in `brush.list`
+order: the brushes go before the first brush of that type at or after `to` that is not moving, and after the last
+brush of that type when there is none or `to` is left out. A move that keeps the order records no undo step.
+Brushes of different types, or a `type` other than theirs, are refused with "brushes move only within their type".
 Document colour mode: `file.new {colorMode: "cmyk"}` starts a CMYK document with CMYK default swatches, and RGB
 colors applied to it (`paint.setFill`/`setStroke` colors, gradient stops and freeform points, `paint.editGradient`
 stops, `paint.freeform.addPoint`/`setPoint`, `paint.sampleColor`, `object.mesh.setPointColor`/`addLine`, the