popover: House any added child widget within a second child
popover: House any added child widget within a second child

This second child is used to control the sizing of the popover and allows us
to manipulate how much space is used on screen, without worrying about
someone changing the popover or popover-child margins.

Signed-off-by: Ikey Doherty <d8d992cf0016e35c2a8339d5e7d44bebd12a2d77@solus-project.com>

ikeydoherty/ui-tests,ikeydoherty/ui-tests