[plasma-nm] Refresh wifi networks as fast as possible

Summary:
Added a retry mechanism when scanning fails, to prevent waiting unnecessarily long for the next scan.

This usually happens when closing and reopening the applet, or when a scan was started by another component before opening the applet.

Test Plan:
* First enable debug logging, there is no UI feedback
* Tested opening/closing the applet every now and then: wifis are scanned as early as NM allows
* Tested switching to the NM KCM, then switching to the applet, then to the KCM again: wifis are always scanned as early as allowed

Reviewers: jgrulich, ngraham, #plasma

Reviewed By: jgrulich

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18896