Currently, in method_list_games (`ListGames` on the bus), when
sd_bus_message_append fails, we break the loop but then we were
not reporting the error (contained in r) back to call.
For each registered game, export an object on the bus under the
path "/com/feralinteractive/GameMode/Games/<pid>" with an dbus
interface of ""com.feralinteractive.GameMode.Game". The interface
currently provides to properties, ProcessId and Executable.
Additionally add the ListGames method and the GameRegistered,
GameUnregistered signals to the com.feralinteractive.GameMode
interface.
