Keyboard: fix key handling/eating logic

As noticed by leon-p, last refactorings made river send a release event
to the client even if the press event has been eaten. In addition, the
introduction of input method support means that we need to remember
*why* we've eaten the key.

Also make KeycodeSet more strict: i am not aware of any case when a
keyboard could have the same key pressed twice (specifically, keyboard
groups have this handled in wlroots), so make the behavior follow a
smaller set of possible scenarios.

river/Seat.zig

@@ -307,6 +307,7 @@ fn keyboardNotifyEnter(self: *Self, wlr_surface: *wlr.Surface) void {
     if (self.wlr_seat.getKeyboard()) |wlr_keyboard| {
         var keycodes = KeycodeSet{
             .items = wlr_keyboard.keycodes,
+            .reason = .{.none} ** 32,
             .len = wlr_keyboard.num_keycodes,
         };