Libraries panel: the list scrolls; items and groups reorder by dragging, items drag into groups

The list fills the panel down to its bottom bar and scrolls, also while a drag nears its top or
bottom edge (in the dock and in a flyout, as the Layers panel does).

Dragging in the list:
- an item (colour, text style or graphic) dropped on another of its kind lands just before or
  after it, by the pointer's half, an accent line showing where, and joins that item's group;
- dropped elsewhere on a group (its header too, even collapsed) it goes into that group, and on
  the ungrouped items out of its group, the section outlined; with every item grouped, a
  "Drop here to take it out of its group" strip shows while one is dragged;
- a group dragged by its header goes before or after another group.
Colours and graphics still drag onto art and the canvas as before.

Engine: library.moveItem takes `before` / `after` (an item of the same kind) to place the item
in its kind's order, which the panel shows; the new library.moveGroup {group, to} moves a group.
Text styles drag as PanelDrag::LibraryItem, groups as PanelDrag::LibraryGroup, and a library
colour's Paint drag names its library item, so the panel tells it from a swatch dragged in.

Tests: reordering and moving in the engine, the commands in the library fuzz test, and panel
tests that drop styles and group headers and check the list scrolls and scrolls while dragging.
Strings translated in the 23 catalogs; docs/mcp.md and the parity row updated.

docs/mcp.md

@@ -1564,9 +1564,13 @@ Items can be put in user-named groups, which the panel shows as collapsible sect
 search field filters every item by name; `library.get` lists them all to filter). `library.createGroup {library?,
 name?, items?: [{kind, item}]}` makes one (a name another group has, ignoring case, gets a number) with those items
 moved into it; `library.renameGroup {library?, group, name}` renames one; `library.deleteGroup {library?, group}`
-deletes one, its items staying in the library ungrouped; and `library.moveItem {library?, kind, item, group?}` moves
-an item into a group (without `group`, out of its group). An item is in one group at most; `library.get` lists the
-groups as `groups: [{name, items: [{kind: "color"|"charStyle"|"paraStyle"|"graphic", item}]}]` (a graphic by its id).
+deletes one, its items staying in the library ungrouped; and `library.moveItem {library?, kind, item, group?, before?
+| after?}` moves an item into a group (without `group`, out of its group) and, with `before` or `after` (another item
+of its kind), just before or after that item in the order its kind is listed and shown in. `library.moveGroup
+{library?, group, to}` moves a group to place `to` of the list (0: first; past the end: last). Dragging items and
+group headers in the panel runs these. An item is in one group at most; `library.get` lists the groups as `groups:
+[{name, items: [{kind: "color"|"charStyle"|"paraStyle"|"graphic", item}]}]` (a graphic by its id), and each kind's
+items in the order the panel shows them.
 
 `library.export {library?, path?}` writes a library (its colours, styles, graphics with their art and thumbnails, and
 its groups) as a `.vclibrary` file, the JSON the library folder keeps (without `path`: `{data}`, the file's text).