dbus: add 'scanning' property

When the supplicant is connected and performs a scan, it doesn't enter
WPA_SCANNING state for various reasons.  However, external programs
still need to know that the supplicant is scanning since they may not
wish to perform certain operations during a scan (as those operations
will likely fail or yield incorrect results).  Add a 'scanning' property
and signal to the supplicant dbus interface to allow clients to
synchronize better with the supplicant when it scans.

Signed-off-by: Dan Williams <dcbw@redhat.com>