label only selectable after we have the transfer code

dropship.glade

@@ -30,7 +30,6 @@
                     <property name="visible">True</property>
                     <property name="can_focus">False</property>
                     <property name="label" translatable="yes">Drag a file to send</property>
-                    <property name="selectable">True</property>
                   </object>
                   <packing>
                     <property name="expand">False</property>

dropship.py

@@ -144,6 +144,7 @@ class DropShip:
 
         line = await self.read_lines(process.stderr, "wormhole receive")
         code = line.split()[-1]
+        self.drop_label.set_selectable(True)
         self.drop_label.set_text(code)
 
         self._pending.append(PendingTransfer(code))