Expose `sender_pid` from notifications

[MicroTransactionsMatterToo]

Issue description

As best I can tell, dunst doesn't expose the sender-pid data from notifications anywhere. This makes matching to a specific window difficult if not impossible, since it's the only piece of information that concretely ties a notification to a specific process.

It'd be nice if this was exposed the same way that appname and other D-Bus information is

[bynect]

looking back at this, I'm wondering what use could the pid be in the dunstrc since the pid will change. the only usage I can think of is when #1348 will be added