Add property lastScanTime and lastRequestTime to WirelessDevice

Summary:
To properly use the WirelessDevice::RequestScan, we need to listen to LastScan property change.

lastScanTime reflects the time when we receive a LastScan property change.
lastRequestScanTime reflects the time when we last requested a scan.

See: https://developer.gnome.org/NetworkManager/stable/gdbus-org.freedesktop.NetworkManager.Device.Wireless.html#gdbus-method-org-freedesktop-NetworkManager-Device-Wireless.RequestScan

Reviewers: jgrulich

Reviewed By: jgrulich

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23576