KeyboardDragListener cares more about left arrow then right arrow

[zepumph]

Discovered while reviewing phetsims/my-solar-system#86.

Because of the implementation of dragging, left will take precedent if both are down. Instead of this block, should we care about which has been down for the least amount of time? That is probably the most idiomatic.

scenery/js/listeners/KeyboardDragListener.ts

Lines 792 to 805 in e615c88

	if ( this.leftMovementKeysDown() ) {
	deltaX = -delta;
	}
	else if ( this.rightMovementKeysDown() ) {
	deltaX = delta;
	}
	if ( this.upMovementKeysDown() ) {
	deltaY = -delta;
	}
	else if ( this.downMovementKeysDown() ) {
	deltaY = delta;
	}

@jessegreenberg have we discussed this before? I can't tell if it is a bug.

[zepumph]

There are two possible ways we could have fixed this inconsistency.

1. Both directions down cancel each other out
2. If both down, take the value from the "most recent" direction down.

(1) is much much easier to implement, so that is where we will start. We also like it quite a bit. @jessegreenberg can you recommend if you'd prefer to see (2)? Please review.

[zepumph]

Tested in MSS, BASE, and Friction

[jessegreenberg]

Thanks! I have flip-flopped on this, I changed it last in #1460. I like your change. 1) also seems most expected for 2D dragging and gaming contexts. Closing.