Windows GDI IME: native JP/CJK candidate UI and text input restored

[victor_18993]

Fixed in SVN r12178.

The Windows GDI IME handling has been corrected.

The issue was caused by the Windows backend consuming WM_IME_SETCONTEXT without forwarding it to DefWindowProc. On current Windows 11 this not only suppressed the native JP/CJK candidate window, but could also prevent Japanese IME text from being committed into Simutrans edit fields.

The fix restores the native Windows IME handling while keeping the composition mask used by the older Windows 10 1809 workaround.

Verified before integration on current trunk:

- native Japanese candidate window is displayed and correctly positioned;
- Japanese text can be converted and committed correctly in multiple Simutrans dialogs;
- normal Latin keyboard input is unchanged;
- clipboard behaviour is unchanged;
- full automated test suite passes 286/286 before and after the fix.

Integrated as:

r12178 — FIX: restore native IME handling on Windows GDI

This change only affects the Windows GDI input backend. There are no savegame, network or pak-format changes.