Add missing files to POTFILES.in; add a translation hint

POTFILES.in was missing some files.

"Activities" is not easily translatable into some languages, e.g. Russian,
so suggest using the word for "Overview" instead.

js/ui/panel.js

@@ -267,8 +267,8 @@ Panel.prototype = {
             this._rightBox.allocate(childBox, flags);
         }));
 
-        /* left side */
-
+        /* Button on the left side of the panel. */
+        /* Translators: If there is no suitable word for "Activities" in your language, you can use the word for "Overview". */
         this.button = new Button.Button(_("Activities"), PANEL_BUTTON_COLOR, PRESSED_BUTTON_BACKGROUND_COLOR,
                                         PANEL_FOREGROUND_COLOR, DEFAULT_FONT);
         this.button.actor.height = PANEL_HEIGHT;

po/POTFILES.in

@@ -1,8 +1,12 @@
 data/gnome-shell.desktop.in.in
-js/ui/panel.js
+js/ui/appDisplay.js
+js/ui/appIcon.js
 js/ui/dash.js
+js/ui/panel.js
 js/ui/overview.js
+js/ui/appIcon.js
 js/ui/runDialog.js
+js/ui/widget.js
 src/gdmuser/gdm-user.c
 src/shell-global.c
 src/shell-status-menu.c