Add an option to filter out Proprietary snaps (and Flatpaks) from GNOME Software

[Ads20000]

Currently, there's no options to filter out proprietary snaps (and, I suppose, Flatpaks?) from Software. When I click on the application drop-down for Software, it links to Software & Updates, which I suppose is where Software would be filtered. Please could an option to filter out proprietary snaps (just as there are options to filter out proprietary Debs by only enabling the main and universe pockets) be provided? The option for snaps should only be visible when gnome-software-plugin-snap is installed and the option for Flatpaks only visible when gnome-software-plugin-flatpak is installed.

Like with the Debs though, perhaps this should also affect the command line, if a `snap install` is attempted for a proprietary snap, or one attempts to install a proprietary Flatpak from a remote, the command line should print something like 'The ability to install proprietary snaps' or Flatpaks 'has been disabled so the snap' or Flatpak 'will not be installed. If you would like to install the proprietary snap' or Flatpak 'then change the setting for proprietary snaps' or Flatpaks 'in Software & Updates'.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: software-properties-gtk 0.96.24.32.3
ProcVersionSignature: Ubuntu 4.15.0-22.24-generic 4.15.17
Uname: Linux 4.15.0-22-generic x86_64
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
CurrentDesktop: communitheme:ubuntu:GNOME
Date: Fri Jun 1 10:25:23 2018
InstallationDate: Installed on 2017-08-03 (301 days ago)
InstallationMedia: Ubuntu 17.04 "Zesty Zapus" - Release amd64 (20170412)
PackageArchitecture: all
SourcePackage: software-properties
UpgradeStatus: Upgraded to bionic on 2018-04-22 (40 days ago)