Add current USB protection status

This is related to the GNOME-Control-Center MR gnome-control-center!366 (merged), the gnome-settings-daemon MR gnome-settings-daemon!75 (merged) and the gsettings-desktop-schemas MR gsettings-desktop-schemas!15 (merged).

I talked about this in Planet GNOME. First post here https://ryuzakikk.github.io/gnome/internship-preparation/

When the USB devices protection is active we show an icon in the top indicator panel. If the required USBGuard is not available we do not show the icon.

As shown in the two images above we are currently using a generic "external device" icon. Probably we will need a custom made icon that indicates "USB protection is on". So something related to a lock and a USB connector.