GTK Popover sample

popover_sample.c

/* 
 * Build:
 * gcc $(pkg-config --cflags gtk+-3.0) popover_sample.c -o popover_sample $(pkg-config --libs gtk+-3.0)
 */

#include <gtk/gtk.h>

int main (int argc, char *argv[])
{
  GtkWidget *window;
  GtkWidget *button, *popover, *vbox;

  gtk_init (&argc, &argv);

  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  gtk_window_set_position (GTK_WINDOW (window), GTK_WIN_POS_CENTER);
  gtk_container_set_border_width (GTK_CONTAINER (window), 5);
  gtk_window_set_default_size (GTK_WINDOW (window), 640, 480);

  button = gtk_menu_button_new ();
  gtk_button_set_image (GTK_BUTTON (button), gtk_image_new_from_icon_name ("open-menu-symbolic", GTK_ICON_SIZE_BUTTON));
  gtk_widget_set_valign (button, GTK_ALIGN_CENTER);
  gtk_widget_set_halign (button, GTK_ALIGN_CENTER);
  gtk_container_add (GTK_CONTAINER (window), button);

  vbox = gtk_box_new (GTK_ORIENTATION_VERTICAL, 0);
  gtk_box_pack_start (GTK_BOX (vbox), gtk_label_new ("Hello World 1"), FALSE, FALSE, 0);
  gtk_box_pack_start (GTK_BOX (vbox), gtk_label_new ("Hello World 2"), FALSE, FALSE, 0);
  gtk_box_pack_start (GTK_BOX (vbox), gtk_label_new ("Hello World 3"), FALSE, FALSE, 0);

  popover = gtk_popover_new (button);
  gtk_container_add (GTK_CONTAINER (popover), vbox);
  gtk_menu_button_set_popover (GTK_MENU_BUTTON (button), popover);
  gtk_widget_show_all (popover);

  g_signal_connect (G_OBJECT (window), "destroy", G_CALLBACK (gtk_main_quit), NULL);

  gtk_widget_show_all (window);

  gtk_main ();

  return 0;
}

xfce4-sample-plugin.diff

diff --git a/panel-plugin/sample.c b/panel-plugin/sample.c
index 97a0cc6..8927da2 100644
--- a/panel-plugin/sample.c
+++ b/panel-plugin/sample.c
@@ -135,7 +135,7 @@ sample_new (XfcePanelPlugin *plugin)
 {
   SamplePlugin   *sample;
   GtkOrientation  orientation;
-  GtkWidget      *label;
+  GtkWidget      *button, *popover, *vbox;
 
   /* allocate memory for the plugin structure */
   sample = g_slice_new0 (SamplePlugin);
@@ -158,13 +158,22 @@ sample_new (XfcePanelPlugin *plugin)
   gtk_container_add (GTK_CONTAINER (sample->ebox), sample->hvbox);
 
   /* some sample widgets */
-  label = gtk_label_new (_("Sample"));
-  gtk_widget_show (label);
-  gtk_box_pack_start (GTK_BOX (sample->hvbox), label, FALSE, FALSE, 0);
+  button = gtk_menu_button_new ();
+  gtk_button_set_image (GTK_BUTTON (button), gtk_image_new_from_icon_name ("open-menu-symbolic", GTK_ICON_SIZE_BUTTON));
+  gtk_box_pack_start (GTK_BOX (sample->hvbox), button, FALSE, FALSE, 0);
+  gtk_widget_show (button);
 
-  label = gtk_label_new (_("Plugin"));
-  gtk_widget_show (label);
-  gtk_box_pack_start (GTK_BOX (sample->hvbox), label, FALSE, FALSE, 0);
+  vbox = gtk_box_new (GTK_ORIENTATION_VERTICAL, 0);
+  gtk_box_pack_start (GTK_BOX (vbox), gtk_label_new ("Hello World 1"), FALSE, FALSE, 0);
+  gtk_box_pack_start (GTK_BOX (vbox), gtk_label_new ("Hello World 2"), FALSE, FALSE, 0);
+  gtk_box_pack_start (GTK_BOX (vbox), gtk_label_new ("Hello World 3"), FALSE, FALSE, 0);
+
+  popover = gtk_popover_new (button);
+  gtk_container_add (GTK_CONTAINER (popover), vbox);
+
+  gtk_widget_show_all (popover);
+
+  gtk_menu_button_set_popover (GTK_MENU_BUTTON (button), popover);
 
   return sample;
 }

I still don't understand what you really want to accomplish, seems like you want to replicate Cinnamon menu or Elementary OS' app launcher. If that's the case you can take a look at their code, but I'll be surprised if that's easily supported by GTK, perhaps it's a popover that belongs to the panel itself, not a plugin in its own process.