In some cases users have to insert special characters, for example polish alphabet contains some extra letters or umlauts of german alphabet or symbols like a celsius sign “°C”. I have developed this functionality in form of plugin. It inserts a trigger to field which popups the panel with special symbols. The plugin is implmented in two files, the plugin and popup window.

and the popup:

The symbols can be listed in as an array or function.

Fiddle: