add "timeshift" to the tempdir names

[ygerlach]

I noticed some aged folders in /tmp on my computer.

I first didnt know where they come from. I found a file "0" and "status" inside all of them. So it seems they are from timeshift. To let users know what program created those files (and give the some information to decide if they want to delete those files), I added the string "timeshift" to those folders.

It also seems like the "exitCode file" was not used. But maybe i have overlooked some place?

I think the whole class is a bit weird. Instead of writing every little command into a script and executing that, it might be better to execute some shorter commands directly. Instead of watching the existence of the "status" file its probably beneficial to check for the pid of the specific progress directly. This looks like a job for https://valadoc.org/glib-2.0/GLib.ChildWatchSource.html

[ygerlach]

I have now added some more possible temp directories. Those should provide valid fallback if root is full and the computer has other another drive mounted on /var as some do.
I also choose to include /dev/shm as fallback. On "recent" linux distros this is expected to be a mounted tmpfs and should therefore never be affected by a full drive (only full memory).

This now
Fixes #397