Ability to block installation of untrusted software not in trusted app stores

[Clinton H]

1) Ubuntu 19.10
2) Gnome Settings 1:3.34.1-1ubuntu2
3) Ability to block installation of untrusted software not in trusted
app stores(Ubuntu Software Center, Steam, etc). The default setting
should be to block installation of untrusted software. If a user
chooses to allow installation of software from untrusted sources, a
warning should be displayed and ClamAV should be installed to protect
the user from malicious software. If Ubuntu becomes more popular,
users might get malicious .deb email attachments. Inexperienced users
might run the malicious .deb file and enter their root password.
4) Installation of untrusted software not in trusted app stores is not
blocked by default.