Compose key support missing in QtWayland

Qt does not support compose key even if it's set in the keymap. This makes it impossible to e.g. input letters like ö, ä, ü, ß with an English keyboard. Those letters just were entered through the compose key on an Xwayland window.

Steps to reproduce:
1. Use a Wayland session (e.g. Plasma 5.7)
2. Setup a keymap with compose key support (e.g. systemsettings -> Input Devices -> Keyboard -> Advanced -> select a "Position of compose key")
3. Open a Qt application with --platform wayland
4. Try to enter text with the compose key
5. Open same application with --platform xcb
6. Try to enter text with compose key

Actual Result: input is wrong in case of Qt window
Expected Result: same text entered whether it's xcb or wayland

Please note that GTK+ on Wayland handles this correctly.