Add fcitx5 dbus interface to fcitx abstraction
Similar reference in snapd https://github.com/snapcore/snapd/pull/12924
Reference upstream implementation:
https://github.com/fcitx/fcitx/blob/master/src/frontend/ipcportal/ipcportal.h https://github.com/fcitx/fcitx5/blob/master/src/frontend/dbusfrontend/dbusfrontend.cpp