Fix #1876: Artboard Options has X and Y with a reference point

Artboard Options showed Name, Width and Height only: the generic form
hides x and y, which in the shape dialogs are the click position. The
artboard's position could be dragged but not typed.

The dialog now draws its own rows after the form's, in the same grid:
the reference point locator the Transform panel, the Control bar and
Properties share, then X and Y. They show where the reference point
lies, counted from the ruler origin (#1212), as the Transform panel and
the Control bar show an artboard's position. A typed X or Y moves the
artboard so that its reference point lands there, and a new Width or
Height keeps the reference point in place. The fields still hold the
top-left corner in document points, so OK runs artboard.setProps as
before, in one undo step. The shape dialogs keep their x and y hidden.

form::grid_then lets a dialog add rows to the generic grid.

docs/mcp.md

@@ -3858,7 +3858,9 @@ desktop app. The Flare tool's are its Flare Tool Options (dialog `flareOptions`)
 `direction` (°), and `raysOn` and `ringsOn` (the Rays and Rings checkboxes; off, the flare has none). OK keeps them
 (`tool.setOption {tool: "flare", values}`) and the next flare dragged out uses them; a click with the tool opens the
 same dialog, and its OK also draws a flare there. The Artboard tool's are the active artboard's Artboard Options
-(dialog `artboardOptions`; the Artboard tool's active artboard while it is in use, else the window's), a graph tool's
+(dialog `artboardOptions`; the Artboard tool's active artboard while it is in use, else the window's; its `x` and `y`
+are the artboard's top-left corner in document points, while the dialog shows X and Y at the reference point the
+Transform panel uses, counted from the ruler origin), a graph tool's
 the selected graph's Graph Type (an error without one), a Symbolism tool's the Symbolism Tools Options (dialog
 `symbolismOptions`: `diameter` in points, `intensity` and `density` 1–10, one brush for the eight tools) and the
 Magic Wand tool's its panel. `ui.newArtboard` (Alt-click on the Artboards panel's New Artboard) opens Artboard Options