Use native Windows API for converting keystrokes to characters (take 2)

Forward-port of !991 (merged) to GTK4

This differs from !4986 (merged) in that this uses a gunichar to store the final output character composed from a keystroke sequence. Keyboard layouts tipically are made to output just one Unicode character, but it's unclear whether we can have layouts that output something more complex, like e.g. compose sequence characters, following a commit keystroke. See #5172 (closed)

Fixes #2944 (closed)