Fix a segfault when going back from the "one at a time" layout and other related bugs

Thanks to Brian Jackson <iggy@theiggy.com> (iggy on IRC) for a bugreport.

REVIEW: 110736
GUI: fix the size of the inner widgets when exiting the "one at a time" mode
GUI: use RAII for the one-at-time layout instead of manual slot setup and other stuff