The only point of using a custom container here was to prevent StBoxLayout
from enforcing the wrong request mode based on the orientation. With that
issue fixed, we can simplify the checkbox widget significantly.
https://bugzilla.gnome.org/show_bug.cgi?id=703811
StBoxLayout currently does not handle height-for-width children
correctly under some circumstances. As a work-around, hard-code
a label height of two lines of text, which should work for most
locales in the one place the widget is currently used.
https://bugzilla.gnome.org/show_bug.cgi?id=672543