Fix multiple focus issues (#426)

* Handle well the situation when mfp.st.focus is set to
a non-existing element (set focus to mfp.wrap instead).

* Fix focus wrapping when mfp.st.focus is set. Focus now wraps to
mfp.wrap instead to mfp.st.focus, which may not not be at the top of
the popup, effectively hiding focusable elements above it from tab
navigation.