Display mounted Archives, look for existing mounts on startup

[m3nu]

(copied from #129)

The following improvements to the mount-feature are suggested:

• Add some kind of mounted icon to the archive table when an archive is mounted, so the user can quickly see which are mounted.
• Since the dictionary of mount points per archive is stored in-process, it's lost when the user restarts Vorta. Which means that upon restart, all archives appear as unmounted and the user has to resort to the command line to unmount them, which is not nice. I would like to use psutil to parse the running commands on the system, search for "borg mount" commands and extract the archive names/mount points from there so the information used by Vorta is always correct (I would love to see any other way to get the archive associated with a mount point, but I'm afraid I couldn't find any other way to get that information but to parse command lines).

[m3nu]

• per-archive mount icons can be added using the QToolButton element.
• checking for existing mounts could be done when the window is opened or the profile changed. It just needs to be async to avoid blocking the UI or delaying the startup. So the user would see the correct mount states after a second or so. If the borg command is called to list mounts, I'd implement it as subclass of BorgThread, which is used to wrap borg commands.

[ThomasWaldmann]

Hmm, there is a slightly different understanding here:

• "mounted icons" = icons showing mount status
• "mount icons" = icons triggering a mount action

As it is not possible to use mount icons to mount multiple archives, I'ld suggest keeping the mount action button below the list and just show mount status.

[m3nu]

I'd suggest keeping the mount action button below the list and just show mount status.

Also better performance-wise. When there are a lot of archives, there would be a lot of buttons to set up. The Delete button from #119 could go below as well.

Currently buttons that (mostly) act on a single archive are on the left side and those for the whole repo on the right.