Support the portal in the gtk im modules

Support the portal in the gtk im modules

This adds a new way to create an IbusBus, ibus_bus_new_async_client().
This returns an object that is not guarantee to handle any calls
that are not needed by a client, meaning CreateInputContext and
handling the input context.

If you are running in a flatpak, or if IBUS_USE_PORTAL is set, then
instead of talking to the regular ibus bus we connect to
org.freedesktop.portal.IBus on the session bus and use the
limited org.freedesktop.IBus.Portal interface instead of the
org.freedesktop.IBus interface.

This allows flatpaks (or other sandbox systems) to safely use
dbus clients (apps).

BUG=https://github.com/flatpak/flatpak/issues/675

Committed: 35ce62474fa97a5460d72c360943700a413a07ae

[fujiwara, 2017-08-30 02:49:33]

This patch is the second of two which works in Flatpak.

Alexander Larsson provided the patches:
https://github.com/alexlarsson/ibus/tree/ibus-portal-2

[Peng, 2017-08-30 13:38:13]

lgtm

[fujiwara, 2017-08-31 03:03:55]

Committed patchset #1 (id:1) manually as
35ce62474fa97a5460d72c360943700a413a07ae (presubmit successful).