Aerc usually used the path []int{1} if it didn't know what the proper path is.
However this only works for multipart messages and breaks if it isn't one.
This patch removes all the hard coding and extracts the necessary helpers to lib.
Especially if one tries to interact with all marked messages there could be
the case that not all headers are fetched yet, hence the messageInfo is still nil.
This segfaults a lot of commands which in principle only need the uid to complete.
If we switch to uids, this issue can be alleviated for those commands.
