Add a 'on_panel_activated()' method into the panel interface

We have added 'on_view_activated()' method into view interface
previously, so for consistency, we do the same for panel. Let
the panel have a chance to do some work after activated.

gitg/gitg-window.vala

@@ -509,6 +509,12 @@ public class Window : Gtk.ApplicationWindow, GitgExt.Application, Initable, Gtk.
 	private void on_panel_activated(UIElements elements,
 	                                GitgExt.UIElement element)
 	{
+		GitgExt.Panel? panel = (GitgExt.Panel?)element;
+
+		if (panel != null)
+		{
+			panel.on_panel_activated();
+		}
 	}
 
 	private void activate_default_view()

libgitg-ext/gitg-ext-panel.vala

@@ -37,6 +37,14 @@ namespace GitgExt
  */
 public interface Panel : Object, UIElement
 {
+	/**
+	 * Give the panel itself a chance to perform some actions after being
+	 * activated.
+	 *
+	 * @return void
+	 *
+	 */
+	public abstract void on_panel_activated();
 }
 
 }

plugins/diff/gitg-diff.vala

@@ -75,6 +75,10 @@ namespace GitgDiff
 			owned get { return "diff-symbolic"; }
 		}
 
+		public void on_panel_activated()
+		{
+		}
+
 		private void on_selection_changed(GitgExt.ObjectSelection selection)
 		{
 			selection.foreach_selected((commit) => {

plugins/files/gitg-files.vala

@@ -82,6 +82,10 @@ namespace GitgFiles
 			owned get { return "system-file-manager-symbolic"; }
 		}
 
+		public void on_panel_activated()
+		{
+		}
+
 		private void on_selection_changed(GitgExt.ObjectSelection selection)
 		{
 			selection.foreach_selected((commit) => {