Add "Wi-Fi Access Points Visible" to main display

This allows the user to see if any access points are currently visible,
as well as whether the number is changing which prevents stalled polls
from continuing.
Remove redundant initializations in WifiScanner constructor
Reset mVisibleAPs to 0 in stop()

MozStumbler isn't scanning after a stop(), so the reported number
would be stale.
Merge pull request #263 from keverets/visible-wifi-count

Add "Wi-Fi Access Points Visible" to main display