Before requesting a scan, check the time threshold

Summary:
Currently we don't follow the documented way to check requestScan results and if it has succeeded.
This ends up generating, retries and warning such as :
```
plasma-nm: Wireless scan on "wlp2s0" failed: "Scanning not allowed immediately following previous scan"
```

This patch checks if the device has not finished a scan within the last 10 seconds and that the last scan finished before allowing more scans to be requested.

RequestScan Doc :
https://developer.gnome.org/NetworkManager/stable/gdbus-org.freedesktop.NetworkManager.Device.Wireless.html#gdbus-method-org-freedesktop-NetworkManager-Device-Wireless.RequestScan
10 seconds request scan threshold in NetworkManager :
https://github.com/NetworkManager/NetworkManager/blob/master/src/devices/wifi/nm-device-wifi.c#L1204

Depends on D23576

Reviewers: #plasma, jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23578