region: Keep format chooser labels centered

In order to prevent the labels from moving around when hiding and
showing the check mark we need to keep the check mark always visible
and instead tweak its opacity.

We also add an always transparent widget the same size as the check
mark on the other side of the label to make sure that the label is
globally centered.

https://bugzilla.gnome.org/show_bug.cgi?id=695536