QtMir/Application: Never drop xwayland connections by closing windows

Explicitly check for the xwayland appId to decide whether to allow closing and deleting the whole Application object when closing the last window.

We never want XWayland to drop, at least until something like an xwayland auto-respawn feature is inside Mir.

Right now this keeps the XWayland icon in the Lomiri launcher, which we might want to explicitly hide from the launcher whenever there's no X11 window up.