When we manage the transient before the main window, the client object's
.transient_for property would stay nil. This happens because the property points
to a window which we don't know (yet) and thus is ignored.
Fix this by remembering the value of WM_TRANSIENT_FOR and checking in
client_manage() if the new client is the "missing window we did not find
before".
Fixes: https://github.com/awesomeWM/awesome/issues/181
Signed-off-by: Uli Schlachter <psychon@znc.in>