Show a flow direction arrow on toggled pumps

[qwaker00]

A pump's facing is hard to read from its sprite alone — which is why vanilla pumps get a flow direction arrow in the first place.
Toggled pumps don't: they reach the world through hidden gauges using linked fluidbox connections and set hide_connection_info, so the engine has no visible connection to draw one on.
In my own playthrough I got into the habit of toggling a pump four times, all the way back around through a plain pump, just to catch a glimpse of its arrow.

Change:

• scripts/valve_arrow.lua renders utility/fluid_indication_arrow on the three large togglable pumps, in alt mode, at the same size and position the engine uses on the plain pumps (centred on the outer edge of the output tile).
• The arrow takes its facing and offset from the pump's own orientation, so it follows rotation with no handler, and the render object dies with the entity.
• Hooked into the existing build/remove paths — no per-tick work.
• migrations/nullius_2.0.12.lua draws the arrows in existing saves.

Small pumps are left out; their graphics already read clearly, but one can easily (with some position adjustments) add arrows to it in future.

Vanilla also shows arrows on hover when alt mode is off. Implementation of that for toggled pumps is hacky, left out for simplicity.

Vanilla pumps to the left, toggle pumps to the right