[Bugfix] make theme menu reset browse context

we try to reuse the browse context to return to folders in the plugin browser

but in the browse_folder() case we want to load the root directory
so reset the current browse context and force the browse
to be reloaded

Change-Id: I416ee40411cdce4e404b6119b33d91ff6ba533c8

apps/menus/theme_menu.c

@@ -325,6 +325,7 @@ int browse_folder(void *param)
         browse.title = str(lang_id);
     }
 
+    tree_get_context()->browse = NULL;  /*bugfix - force root dir reload */
     return rockbox_browse(&browse);
 }