Set max-width-chars property in ibus-setup wrapped GtkLabel.

If GtkWindow has 'resizable' property = FALSE, the wrap in GtkLabel
is not done automatically and need to set 'max-width-chars' property.

TEST=ibus-setup

Review URL: https://codereview.appspot.com/92940043

setup/setup.ui

@@ -902,6 +902,7 @@
                                 <property name="can_focus">False</property>
                                 <property name="xalign">0</property>
                                 <property name="wrap">True</property>
+                                <property name="max_width_chars">74</property>
                                 <property name="label" translatable="yes">&lt;small&gt;&lt;i&gt;The active input method can be switched around from the selected ones in the above list by pressing the keyboard shortcut keys or clicking the panel icon.&lt;/i&gt;&lt;/small&gt;</property>
                                 <property name="use_markup">True</property>
                               </object>