about dialog: Set a max-content-height on credits

This keeps the credits section from making the dialog grow
when there are lots of credits.

https://bugzilla.gnome.org/show_bug.cgi?id=770458

gtk/ui/gtkaboutdialog.ui

@@ -158,6 +158,7 @@
                         <property name="visible">1</property>
                         <property name="hscrollbar-policy">never</property>
                         <property name="shadow-type">in</property>
+                        <property name="max-content-height">60</property>
                         <child>
                           <object class="GtkViewport" id="viewport1">
                             <property name="visible">1</property>