KCoreUrlNavigator: fix header include path, add backward-compat headers
Tested with all of
#include <kio/kcoreurlnavigator.h>
#include <kcoreurlnavigator.h>
#include <KIO/KCoreUrlNavigator>
#include <KCoreUrlNavigator>
in a code linking KIOGui. With warnings enabled/disabled & visinility enabled/disabled.
Same compat header code already used elsewhere in KF, either generating (KDNSSD) or explicit (io_slave_defaults,h). Somebody (tm) should turn this into a ECM macro.