fix menu-item floating buttons that should only be displayed on hover

[charlesBochet]

MenuItems floating icons should only be displayed if the menu item is hovered:

and hovered:

We have two options to implement it:

1. Cleaner with React, have a [isHovered, setIsHovered] state that will be set onMouseEnter and onMouseLeave on MenuItem and isHovered is used to conditionnally display buttons.
2. Add a customClass on buttons and use nested css to apply menuItem hover display none/block

I've used 2) so we can have it tested in the Storybook, I think it's important for a UI component to be properly tested and as we won't touch this component often, it's OK not going with the cleaner approach IMO