Use new Modifiers system for the arc tool

Part of #5839

There is one downside to the general system - using Modifiers::responsive_tooltip with shared shortcuts means you get the very generic description of the shortcut instead of the previous tool-specific tip (Keep aspect ratio instead of make circle or integer-ratio ellipse, snap arc/segment angle). There's a possible improvement to be able to provide text overrides to that method? Dunno if that's worth it.

Tagging @doctormo too on this first one, just to confirm I did the pattern right.