When the kitty keyboard protocol "report all keys as escape codes" mode
was active, composed/IME text (e.g. from dead keys or compose sequences)
was silently dropped.
This happened because the composed text is sent within our GTK apprt
with key=unidentified and no unshifted_codepoint, so no kitty entry was
found and the encoder returned without producing any output. The
plain-text fallback was also skipped because report_all bypasses it.
Send composed text as raw UTF-8 when no kitty entry is found, matching
the behavior of Kitty on Linux for me.
Fixes#10049
fdfc9fea2f