D-Bus bus-activated service support

[axelkar]

Description

dbus-daemon gives us support to load session service files from these HM-accessible directories:

• $XDG_DATA_HOME/dbus-1/services (~/.local/share/dbus-1/services, maybe via activation hook?)
• $XDG_DATA_DIRS/*/dbus-1/services (~/.nix-profile/share/*/dbus-1/services)

It seems that it's very possible to add bus-activated services to fix issues like #1454 (comment)

[axelkar]

You can actually just put them in home.packages and they'll appear because all files and directories get linked. Now we just need a way to reliably propagate XDG_DATA_DIRS to dbus-daemon (and restart the daemon if the variable is changed).

Edit: Nevermind the environment variable stuff. NixOS automatically add all share directories of profile locations to XDG_DATA_DIRS.