Add polkit action for Wi-Fi scans

Previously, Wi-Fi scans uses polkit action "org.freedesktop.NetworkManager.network-control". This is introduced in commit 5e3e19d0. But in a system with restrict polkit rules, for example "org.freedesktop.NetworkManager.network-control" was set as auth_admin. When you open the network panel of GNOME Control Center, a polkit dialog will keep showing up asking for admin password, as GNOME Control Center scans the Wi-Fi list every 15 seconds.

Fix that by adding a new polkit action "org.freedesktop.NetworkManager.wifi-scan" so that distributions can add specific rule to allow Wi-Fi scans.