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
Test plan: Open kwin debug console, and examine the events you get when
- Press shift
- Press 1
- Release 1
- Release shift