Drag to reorder the toolbar and favourites pickers

[marcinz606]

Follow-up to #894. The chosen column of the shared picker now takes internal drags, so an item moves by being dragged where it belongs instead of being walked there one arrow press at a time. Applies to both users of the dialog: Edit Toolbar… and Edit Favourites.

What changed

negpy/desktop/view/widgets/favourites_dialog.py

• _ReorderList — a QListWidget in InternalMove mode with a reordered signal. QListWidget implements an internal move as a remove plus an insert, so rowsMoved never fires; the drop event is the only reliable signal, and the list is read back as the truth afterwards.
• The up/down arrows are gone, and so is the X: the left column's checkboxes already remove an item, and _set_checked existed only to keep the two in step. _rebuild_chosen loses its now-unused select_row argument.
• Both hints gained a leading "Drag to reorder."

docs/USER_GUIDE.md — the Favourites bullet and both canvas-toolbar paragraphs say drag rather than arrow buttons.

Net −43 lines.

Verification

make all green. test_dialog_reads_the_dropped_order_back moves a row the way InternalMove does and asserts the read-back order; test_chosen_list_takes_internal_drags guards the drag configuration itself. The retired _move_up/_move_down/_remove_selected tests are deleted with the code.

Driven offscreen with the real toolbar catalogue: the footer holds only Restore Defaults / Cancel / Apply, the list reports InternalMove + MoveAction, a moved row reads back in the new order, and Restore Defaults still returns the stock row.

The drag gesture itself is Qt's, and simulating a synthetic QDropEvent offscreen does not exercise it faithfully (the event carries no source, so the view declines the internal move), so that part is left to Qt rather than covered by a test that would pass for the wrong reason.