FIX DEV-42093 - Make sure to navigate home if current URL is empty on "Apply"

This change gets rid of the crufty (and confusing) "apply()"
functions from llpanelmediasettings*.  Those functions were
never being called, yet changes were being made in them.

Instead, I added "preApply()" and "postApply()" to each of
these panels, which the floater (who really does the "apply()"ing)
now calls before and after it applies the changes to the
media data.