GTK IME: Override wayland IM module with ibus

Chromium cannot make use of the wayland IM module in GTK because of the
dummy window. So if wayland is picked initially, override that to
attempt using ibus IM module instead.

Users can still override the IM module using GTK_IM_MODULE env var as
before for other IM modules if desired.

Bug: 40801194
Change-Id: Ie304e2746fe5654546976e55c2b132363a6feb77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5631586
Commit-Queue: Orko Garai <orko@igalia.com>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1315602}