Fix an issue where client could not be entered again (#51)

RELEASE_MODIFIERS are now handled server side.

A client exited through CTRL+ALT+SHIFT+SUPER could sometimes not be entered again when the client
did not send a Modifiers (0) event.

Such a client would always respond with Modifiers (RELEASE_MODIFIERS) and immediately cause the sender to
exit again.

To prevent this, a modifier event with all modifiers released is now sent instead when the release modifiers are detected.

src/backend/producer/wayland.rs

@@ -753,10 +753,6 @@ impl Dispatch<wl_keyboard::WlKeyboard, ()> for State {
                         }),
                     ));
                 }
-                if mods_depressed == 77 {
-                    // ctrl shift super alt
-                    app.ungrab();
-                }
             }
             wl_keyboard::Event::Keymap {
                 format: _,