panels: Fix truncated panel names for larger fonts

Note that this fix will not automatically fix translations, which will
need to add soft-hyphens (U+00AD) to their translations themselves, and
will not fix larger fonts for which the split up syllables end up
being bigger than the maximum text width.

It's the best we can do without redesigning the Settings shell, which is
already something planned.

https://bugzilla.gnome.org/show_bug.cgi?id=647087#c13