[feature] Entering Unicode character codes

[alphabitserial]

To test the desired behavior, open any GTK text input and press Ctrl-Shift-U. An underlined u character appears. Then you can enter a Unicode codepoint, and finally hit the space key to end the special input mode and insert the desired character. On a US keyboard layout, this is the only way to input certain characters, as they're not available via compose key sequences.

For example, this is the only way to type the Vietnamese name Thủy on a US layout. I've attached a video demoing the functionality. I believe in GTK this is accomplished via ibus integration.

2024-10-04.13-47-26.mp4