Use MouseEnter/MouseLeave instead of MouseOver/MouseOut

[cgestes]

This applies to a lot of components.

This idea is to improve performances by avoiding useless setState.

Let's take the ListItem example.
If a child component is hovered then the hovered state of ListItem switches back and forth even if the list item is still hovered. This creates a lot of useless rerender.

[cgestes]

React support: https://facebook.github.io/react/docs/events.html

[cgestes]

see #1190

[shaurya947]

#1190 was merged