Being able to close the QR code dialog by pressing the ESC
key would be a handy feature.
By switching from GtkWindow to GtkDialog, the QR code
dialog is closed when pressing ESC.
Closes#2357
Many devices are able to scan a specifically formatted QR code to
connect to a wifi network.
To make sharing of wifi connections easier, it would be helpful to
display such a QR code in the wifi settings.
A button is added to the wifi connection row. This row is shown in
the general wifi settings panel, as well as in the "Known Wi-Fi Network"
dialog. Clicking the button opens an additional dialog, which shows the
QR code.
