[packagekit] Support for trusted packages in InstallPackages

[Sebastian Heinlein]

Hello,

I would like to see support for separating between trusted and untrusted
packages in the InstallPackages method. It should behave in the same way
as in InstallFiles.

By default trusted will be True. If the backend encounters a not trusted
package it should raise BAD_GPG_SIGNATURE error and send the
corresponding RepositorySignatureRequired signal.

The frontend should in this case show a warning about the installation
of untrusted software and provide an alternative button which allows to
run InstallPackages with trusted = False.

Cheers,

Sebastian

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If trusted is set