Allow getting the same export handle multiple times by calling
gdk_wayland_window_export_handle() multiple times. For each time
export() is called, a unexport() must be called to unexport.

When the window is already exported, the exported callback is called
via a idle handler. If there are multiple export() calls, they are
invoked in order either when the handle is received by the display
server, or when the idle callback is invoked.

Calling unexport() will not affect future invokations of the exported
callback, unless all export() calls have their unexport() call count
matched.

https://bugzilla.gnome.org/show_bug.cgi?id=782325