Forward port MR !1063 (Win32 IME fixes) to GTK master

Merged Chun-wei Fan requested to merge mr1063-for-master into master


This is the forward port of MR !1063 (merged) to GTK master, as the issue (#2114 (closed)) that is being fixed on gtk-3-24 applies to GTK master as well.

Bulk of work done by @Isopod (Philip Zander)

With blessings, thank you!

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