cc-popover: add --cc-popover-padding css prop

[pdesoyres-cc]

This property controls the padding to be set on the floating area. By default this padding is set to 0.5em, but in some case, we may want to control the padding css property.

This modification is motivated by the issue #1205 where we want to use the cc-popover to mimic a kind of menu. In this situation, the default padding of 0.5em doesn't fit well.

When there is no padding, we may have issue with overflow and rounded corners, so it is necessary to put the overflow css property to hidden.