add tooltip to explain what ft proxies helps do

src/gtkgui.glade

@@ -1121,6 +1121,7 @@
 	      <child>
 		<widget class="GtkCheckButton" id="use_ft_proxies_checkbutton">
 		  <property name="visible">True</property>
+		  <property name="tooltip" translatable="yes">If checked, Gajim will also broadcast some more IPs except from just your IP, so file transfer has higher chances of working right.</property>
 		  <property name="can_focus">True</property>
 		  <property name="label" translatable="yes">Use file transfer proxies</property>
 		  <property name="use_underline">True</property>