wayland: Do not forget immediately of key press serials on key release

Prior to commit 5dfed8a4, the MetaWaylandKeyboard would always remember the last key press serial, and consider it valid after the key was released, as long as no other key presses/releases happened in between.

That commit improved things so that MetaWaylandKeyboard can track multiple keys being pressed simultaneously, but also changed so that the serial for a key press is immediately forgotten after the key press event was received. This may break in situations like testing or keyboard macros where key press and release is handled in a quick sucession, so the client reaction to the key press (e.g. popping up a menu) might arrive too late.

Add a sort of spiritual successor to this handling, and make keyboard press serials corresponding to the last key up forgotten at the next key press/release received.

Fixes: 5dfed8a4 ("wayland: Preserve serial for all pressed keys") Closes: #3458 (closed)