tooltip: Prevent too-wide tooltip windows

Set the label to expand, so it actually fills
the width that we allocated for it, instead
of shrinking back to the minimum width for
its height.

Fixes: #5521

gtk/ui/gtktooltipwindow.ui

@@ -13,6 +13,7 @@
         </child>
         <child>
           <object class="GtkLabel" id="label">
+            <property name="hexpand">1</property>
             <property name="wrap">1</property>
             <property name="max-width-chars">50</property>
           </object>