Move clearInterface things to mw.LightboxInterface

Also fixes a bug where we didn't clear our arrow key listeners, and VE
would sometimes see lightbox loads accidentally.

+tests for .empty()

Bug: 58107
Change-Id: I58fbd2e5e3cfbf1f2dfe8ab9f6b2900152215ddf
(cherry-picked from commit 1411110b3d585d3b8c4cf62e4e3f891155ac3279)