Open the Color Picker from every colour swatch

[IAmJSD]

Clicking the swatch on Layer Style's Color row did nothing, which is the one thing the control looks like it should do.

The reason it did nothing is that Workspace::modal is a single Option<Modal> and the picker is itself a Modal, so opening it would have thrown away the Layer Style session mid-edit. So modals get a suspend stack: open_color_picker_on pushes the dialog that is already up, and close_modal pops it back rather than clearing, whether the picker was OK'd or cancelled. modal stays the top of the stack, so the dozen places that read it -- the renderer, the key routing, the "is a dialog up" guards -- carry on unchanged. open_modal clears the stack, so a dialog launched from the menus still replaces everything.

Three swatches wanted this, not one:

• Layer Style's per-effect Color, via ColorTarget::StyleEffect(key). OK writes through set_color and re-previews the layer, the same path the sliders use, so the canvas keeps up.
• Gradient Overlay's From and To, which had no swatches at all -- only the "From = FG" / "To = FG" buttons, so the two colours were reachable only by way of the foreground. They ride the same variant under dotted keys, gradient_overlay.from and .to, since they are ends of a gradient rather than "the effect's colour" and have no slot in color_of.
• Select > Color Range's Sampled, which needs its own variant because it writes back to a different modal.

Driven headlessly to check the parts that are easy to get wrong: OK restores the parent dialog with every other control intact, Cancel and Escape restore it with the colour unchanged, and the parent's own OK/Cancel still commit and revert.