HTTPRequest/pkg::Install: do not replace callbacks.

Keep a list of callbacks to allow registering multiple callbacks
to the same event. This is consistent with eg. jQuery.Deferred
and is needed for example to open multiple dialogs showing the
progress of installing a package at the same time.

simgear/io/HTTPRequest.cxx

@@ -39,7 +39,7 @@ Request* Request::done(const Callback& cb)
   if( _ready_state == DONE )
     cb(this);
   else
-    _cb_done = cb;
+    _cb_done.push_back(cb);
 
   return this;
 }
@@ -50,7 +50,7 @@ Request* Request::fail(const Callback& cb)
   if( _ready_state == FAILED )
     cb(this);
   else
-    _cb_fail = cb;
+    _cb_fail.push_back(cb);
 
   return this;
 }
@@ -61,7 +61,7 @@ Request* Request::always(const Callback& cb)
   if( isComplete() )
     cb(this);
   else
-    _cb_always = cb;
+    _cb_always.push_back(cb);
 
   return this;
 }
@@ -321,22 +321,19 @@ void Request::setReadyState(ReadyState state)
     onDone();
     onAlways();
 
-    if( _cb_done )
-      _cb_done(this);
+    _cb_done(this);
   }
   else if( state == FAILED )
   {
     onFail();
     onAlways();
 
-    if( _cb_fail )
-      _cb_fail(this);
+    _cb_fail(this);
   }
   else
     return;
 
-  if( _cb_always )
-    _cb_always(this);
+  _cb_always(this);
 }
 
 //------------------------------------------------------------------------------