Add QR code scanner for connecting to a WiFi network

Through a button in the toolbar, a viewfinder is opened,
which lets you connect to a WiFi network through a
QR code.

Changes the main view to be a StackView so we can add
subpages similar to the Clipboard plasmoid.
2 jobs for !178 with work/kbroulik/scan-qr-code in 2 minutes and 23 seconds (queued for 9 seconds)
latest merge request