Fix window placement on wayland and add close buttons

We explicitly set a parent. We can't assign to
ApplicationWindow.transientParent though, only to
Window.transientParent, so we just call setTransientParent in C++.

src/ui/NhekoGlobalObject.cpp

@@ -6,6 +6,7 @@
 
 #include <QDesktopServices>
 #include <QUrl>
+#include <QWindow>
 
 #include "Cache_p.h"
 #include "ChatPage.h"
@@ -140,3 +141,9 @@ Nheko::openJoinRoomDialog() const
         MainWindow::instance()->openJoinRoomDialog(
           [](const QString &room_id) { ChatPage::instance()->joinRoom(room_id); });
 }
+
+void
+Nheko::reparent(QWindow *win) const
+{
+        win->setTransientParent(MainWindow::instance()->windowHandle());
+}