input-methodunicodeuser interfacex11
Emacs has the insert-char feature: You press a key combination and get a menu containing all Unicode characters with search similar to dmenu and most DE's program launchers.
For example, to enter "∃" (THERE EXISTS), I type <Ctrl-x> 8 <Enter> exists <Enter>.
Is there something similar that works in all X11 programs?
Thanks to @Gilles, I found an answer. So, the problem was with different Input Methods, used by different applications; and IMs in turn use different Compose files — ones used by X are /usr/share/X11/locale/<your-locale>/Compose and ~/.XCompose (the last isn't present by default, but you may create it for custom combinations), and the one used by Qt is in… Well, looks like nobody knows.
The solution is to set xim to be used as default input method by all applications. You can call im-config, and choose there xim as the default input method, or you can manually edit /etc/profile file to add these lines:
export GTK_IM_MODULE=xim
export XMODIFIERS=@im=xim
export QT_IM_MODULE=xim
Not sure about im-config, but for the way with /etc/profile reboot will be needed.
Either you can enable an Compose key, or you can press CtrlShiftU followed by the Unicode hexadecimal number of the character (leading zeroes can be skipped). For example, Æ is CtrlShiftU, then C6. This works on GNOME Terminal, Terminator, Google Chrome and a number of other applications, with Xterm (and according to comments, Emacs) not working.
On GNOME, to enable Compose, go to the Keyboard settings, then Shortcuts → Typing → Alternative Characters Key and map it to something (say, Right Alt). Then you press the Compose key, and - thrice to get an em-dash: —. (The arrows in this paragraph are all Compose->.)
Best Answer
I would think Autokey would do the job as described here. The idea is that Autokey is always running and can accept arbitrary strings as triggers.
As the post describes, you just need to set up Autokey to paste unicode characters and as a trigger it could accept something like /delta, which it then replaces with a δ.