Add a logging category for the backend

Summary:
To get rid of tons of logs like :
using... PackageKit::Transaction(0x55db7f249ab0) "/14940_dbaecdcb"
using... PackageKit::Transaction(0x55db7f33fba0) "/14940_dbaecdcb"
using... PackageKit::Transaction(0x55db7d97b860) "/14941_ecaeeebe"

Test Plan: builds

Reviewers: apol, ngraham, mlaurent

Reviewed By: apol, mlaurent

Subscribers: mlaurent, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26533