Add Z-Index property to front-end components

Z-Index means how much the element is in front of the others. The bigger the value, the bigger is its priority to be in front.

Example: 
Component 1: Z index = -2
Component 2: Z index = 3
Component 3: Z index = 1

The order of back-to-front would be:
Comp 1 -> Comp 3 -> Comp 2