Add-ons screen should show a hint when no add-ons are installed. #3243
Description
Currently the add-ons screen is just blank if there are no add-ons installed and it's not completely obvious what you're expected to do.
I suggest adding a hint that says 'Click "+" to install your first add-on'.
This was less obvious in the Raspbian OS images because we always bundled some add-ons by default, but that is not currently the case with the Docker image or snap package.