GtkSizeGroup is used to make sure widgets in diffirent rows request the
same amount of space  while implementing "Use a single row for each
log entry" in commit 995bacac.
But adding too many widgets to GtkSizeGroup leads to poor performance,
which make scrolling the list in Logs very slow.

Fix that by removing the GtkSizeGroup. As there are not too many widgets
in the "Important" category, keep the category size group to make
category labels request the same amount of space.

https://bugzilla.gnome.org/show_bug.cgi?id=784216

(cherry picked from commit 33297b6d)