Fix Ctrl+PgUp/PgDn shortcut for room switching

Before this change, Ctrl+PgUp/PgDown didn't select the next room, but
actually moved two rooms at once because the signal handler was called
twice. I'm not entirely sure why this happens (it might be a bug in a
lower layer?), but as we don't want to handle the same event elsewhere,
accepting the event doesn't feel wrong and it appears to fix the issue.
So let's just do that.

BUG: 459997

src/qml/Page/RoomPage.qml

@@ -133,8 +133,10 @@ Kirigami.ScrollablePage {
 
     Keys.onPressed: {
         if (event.key === Qt.Key_PageDown && (event.modifiers & Qt.ControlModifier)) {
+            event.accepted = true;
             switchRoomDown();
         } else if (event.key === Qt.Key_PageUp && (event.modifiers & Qt.ControlModifier)) {
+            event.accepted = true;
             switchRoomUp();
         } else if (!(event.modifiers & Qt.ControlModifier) && event.key < Qt.Key_Escape) {
             event.accepted = true;