fix(glightbox.js): remove background focus, fix issue #555

Fixes issue #555: To fix "Blocked aria-hidden on an element because its descendant retained focus..." error on chrome. The issue is the original thumbnail retaining focus despite being aria-hidden, once modal opens. Drawback is the original focus position is lost. Therefore there is a better solution to store original focus position and restore it when modal is closing. Not tested in IE, but sure OK.