The Unicode character table
The Unicode character map allows to insert any Unicode charater handleable by the Java framework into the editor. Just open it through the program's menu bar entry "Settings → Unicode character selection...".
The Unicode character table
The character table always features one page of 256 characters at once. In the upper part, you select the page to show. Then, you insert the character by simply pressing on the button for the required character.
Of course, this input method is not very comfortable if it comes to longer input sequences. But it helps tremendously if you only have to insert some special characters. If you have to type in longer texts, take a look at the keyboard translation feature.