window: Use client PID for meta_window_get_pid()

The shell uses the PID of windows to map them to apps or to find out which window/app triggered a dialog. It currently fails to do that in some situations on Wayland, because meta_window_get_pid() only returns a valid PID for x11 clients.

So use the client PID instead of the X11-exclusive _NET_WM_PID property to find out the PID of the process that started the window. We can do that by simply renaming the already existing meta_window_get_client_pid() API to meta_window_get_pid() and moving the old API providing the _NET_WM_PID to meta_window_get_netwm_pid().