client/gtk2: Fix Atom and Slack for Flatpak
client/gtk2: Fix Atom and Slack for Flatpak

Seems Atom, slack, com.visualstudio.code does not enable
gtk_key_snooper_install() and this issue causes to call
gtk_im_context_filter_keypress instead of calling ibus APIs.

BUG=https://github.com/ibus/ibus/issues/1991

fujiwarat/ibus,ibus/ibus,fujiwarat/ibus,ibus/ibus,fujiwarat/ibus,ibus/ibus,fujiwarat/ibus,ibus/ibus