Important environment variables for using input method on Guix

[Squirrel <1953494819@qq.com>]

Hi everyone,

I see that many users find it hard to use input methods like ibus and 
fcitx in X11 on Guix System. Even though the input method is running, no 
character suggestion panel is shown while typing.

In fact, except for GTK_IM_MODULE, QT_IM_MODULE and XMODIFIERS, the 
environment variables GUIX_GTK2_IM_MODULE_FILE and GUIX_GTK3_IM_MODULE 
also have to be set. For example, the following is in my '~/.myenv', 
which I "source" in '~/.xsession' (I use slim as desktop manager).

     export GTK_IM_MODULE="ibus"
     export QT_IM_MODULE="ibus"
     export XMODIFIERS="@im=ibus"
     export 
GUIX_GTK2_IM_MODULE_FILE=/run/current-system/profile/lib/gtk-2.0/2.10.0/immodules-gtk2.cache
     export 
GUIX_GTK3_IM_MODULE_FILE=/run/current-system/profile/lib/gtk-3.0/3.0.0/immodules-gtk3.cache

In wayland Gnome, it seems that ibus runs perfectly without special 
configurations.

Maybe someone can add the hints in Guix manual? I'm not familiar with 
development tools. :)

Many thanks to https://guix.org.cn/

Reference (in Chinese):

     1. https://guix.org.cn/wiki/input-method/

     2. https://guix.org.cn/wiki/guix-system-gnome-ibus/