add the abbility to switch tabs using [ctrl+tab] and [ctrl+shift+tab]

If true the gsettings key "ctrl-tab-switch-tabs" into "org.mate.pluma"

Closes #211

data/org.mate.pluma.gschema.xml.in

@@ -11,6 +11,11 @@
       <summary>Editor Font</summary>
       <description>A custom font that will be used for the editing area. This will only take effect if the "Use Default Font" option is turned off.</description>
     </key>
+    <key name="ctrl-tab-switch-tabs" type="b">
+      <default>false</default>
+      <summary>Switch tabs with [ctrl] + [tab]</summary>
+      <description>If true, it enables the ability to switch tabs using [ctrl + tab] and [ctrl + shift + tab].</description>
+    </key>
     <key name="color-scheme" type="s">
       <default>'tango'</default>
       <summary>Style Scheme</summary>

pluma/pluma-window.c

@@ -338,6 +338,35 @@ pluma_window_key_press_event (GtkWidget   *widget,
 			if (!g_settings_get_boolean (settings, "use-default-font") && (nsize > 5))
 				g_settings_set_string (settings, "editor-font", g_strconcat (tempfont, tempsize, NULL));
 		}
+
+		if (g_settings_get_boolean (settings, "ctrl-tab-switch-tabs"))
+		{
+			GtkNotebook *notebook = GTK_NOTEBOOK (_pluma_window_get_notebook (PLUMA_WINDOW (window)));
+
+			int pages = gtk_notebook_get_n_pages (notebook);
+			int page_num = gtk_notebook_get_current_page (notebook);
+
+			if (event->keyval == GDK_KEY_ISO_Left_Tab)
+			{
+				if (page_num != 0)
+					gtk_notebook_prev_page (notebook);
+				else
+					gtk_notebook_set_current_page (notebook, (pages - 1));
+				return TRUE;
+			}
+
+			if (event->keyval == GDK_KEY_Tab)
+			{
+				if (page_num != (pages -1))
+					gtk_notebook_next_page (notebook);
+				else
+					gtk_notebook_set_current_page (notebook, 0);
+				return TRUE;
+			}
+		}
+		g_object_unref (settings);
+		g_free (font);
+		g_free (tempsize);
 	}
 
 	if (grand_parent_class == NULL)