add transparency to vector label backgrounds

[pixelzoom]

@kathy-phet reported that she's like to see some transparency in the vector label backgrounds.

[pixelzoom]

We (@arouinfar @ariel-phet @kathy-phet) played with this in realtime, in there was consensus that it looked good.

Here are the current values in VectorAdditionColors:

    ACTIVE_VECTOR_LABEL_BACKGROUND_FILL: 'rgba( 240, 240, 100, 0.8 )', // yellow
    ACTIVE_VECTOR_LABEL_BACKGROUND_STROKE: 'rgba( 230, 230, 90, 0.8 )', // slightly darker yellow
    INACTIVE_VECTOR_LABEL_BACKGROUND_FILL: 'rgba( 235, 235, 235, 0.5 )', // gray
    INACTIVE_VECTOR_LABEL_BACKGROUND_STROKE: 'rgba( 215, 215, 215, 0.5 )', // slightly darker gray

@arouinfar @ariel-phet @kathy-phet please review in master or 1.0.0-dev.36.

[arouinfar]

@pixelzoom I'm happy with how things look in dev.36. @kathy-phet @ariel-phet would you also like to review?

[ariel-phet]

I think all looks quite good. No need to wait for @kathy-phet on this one. Closing