To enter symbols and letters that are not on your keyboard, GTK applications on Linux offer something like Windows ALT codes, but for all Unicode characters, including the latest emojis.
Hold down CTRL-SHIFT-u and you will see an underlined u character that is waiting for input. Type the hexadecimal Unicode value and press space.
http://unicode.org/charts/ is a full collection of charts that list all Unicode characters with their codes.
Here is a cheat sheet for German:
| Letter | Hex code |
|---|---|
| Ä | c4 |
| ä | e4 |
| Ö | d6 |
| ö | f6 |
| Ü | dc |
| ü | fc |
| ß | df |
| € | 20ac |
And here some emojis from the official Unicode chart:
| Emoji | Hex code | Name |
|---|---|---|
| 🙃 | 1f643 | Upside-down face |
| 💣 | 1f4a3 | Bomb |
| 🤡 | 1f921 | Clown face |
Oleander Donovan's IT blog 