Dragging a sprite doesn't bring it to front layer

[jwzimmer-zz]

Expected Behavior

In 2.0, when you drag a sprite, it goes to the front/ top layer.

I'm not sure if we want the 2.0 behaviour in 3.0 or not, since it can be annoying to re-layer your sprite if you just want to adjust the position a bit to see how it looks.

Actual Behavior

Reported by Shruti during the Catch Game Internal Playtest II (6/12/17):

When you drag the sprite on the stage, it doesn't come in front of other sprites. Only the one that's added later remains in front.

Steps to Reproduce

Add a 2nd sprite to the project. Drag Scratch Cat (1st sprite), then let it go when it overlaps with the 2nd sprite. Note it's behind that sprite.

Operating System and Browser

Chrome, Mac

[rschamp]

Tracking in scratchfoundation/scratch-gui#161