Add a slight delay between showing data points and the boxplot

[garrettgman]

The slight delay can reinforce that the boxplot comes from the data points.

[garrettgman]

I added the delay in the boxdotGrob. However, it will be better to separate the points and boxplot into separate grobs which can be controlled at the GUI level - which I will do when I reorganize vit into a package. As it is, vit inserts the delay whenever it draws a boxplot on top of points. Since it redraws everything multiple times during the animations. These delays add up!