🍒 xkb: always update keysym

Previously m_keysym was only updated on key press. This caused issues when multiple keys are pressed at the same time. E.g., if the user presses A, presses B, releases A, releases B, the actual events sent by kwin was A pressed; B pressed; B released; B released.

Also call xkb_state_update_key after xkb_state_key_get_one_sym, as recommended by the libxkbcommon documentation

(cherry picked from commit 38079542)

Are we cherry-picking this to 6.0?