EditablePage: position loading placeholder overlay correctly
It was using negative margins to counteract the ones added to the page, but was then re-parented to something that wasn't the page. If those two things didn't have the exact same geometry, the overlay would be mis-positioned.
Fix this by dropping the re-parenting, and instead manually set the Z to something very high to ensure it always covers the content.
BUG: 488044 FIXED-IN: 6.1.1