[th/firewalld-manager-rename] rename NMFirewallManager to NMFirewalldManager
NMFirewall[d]Manager
is only about firewalld. Also from the API, it is not about firewall (iptables, nftables) in general, as it is concerned about D-Bus' name owner and firewalld zones.
We will have a use for more generic "firewall" code, so rename this class to avoid confusion.