xsettings: Adopt code to look up GTK IM module

Right now, gsd-keyboard and gsd-xsettings have a strange relation where the first sets the gtk-im-module dconf setting for the latter to read the setting and forward it through XSettings.

Since this detection is highly X11 specific, make it happen in the Xsettings daemon itself, from the relevant setting and device presence.

This makes users still able to shoot themselves in the foot by changing the setting, X11 clients able to be told to switch to ibus if it turns out necessary, and Wayland clients unaffected otherwise.

Related: gtk#4443 (closed)