Fix SEGV of ibus-x11 on wayland
Fix SEGV of ibus-x11 on wayland

In case that ibus-x11 is built with gtk3, GdkDisplay can be
GdkWaylandDisplay but not GdkX11Display so GDK_DISPLAY_XDISPLAY()
and GDK_WINDOW_XID() does not work in wayland.

TEST=client/x11/ibus-x11

Review URL: https://codereview.appspot.com/234590043

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