If you press Ctrl+Shift+U on a Linux based system (not sure about UNIX) in any textbox you get a small underscored u. You can then proceed to enter hexadecimal numbers and confirm them with enter. The result you'll get is the Unicode character with the respective hexadecimal number.
For example typing in u6a4c and pressing enter will result in 橌 being written to the text box.
I used a lot of Linux based OSes and everyone of them had this functionality so I wanted to know, does any Linux system come with this and what part of a Linux system is responsible for this feature?