Allow multiple backups at a time

[samuel-w]

Describe the bug
I cannot run multiple backups at a time, even though they are in different repositories.

To Reproduce
Steps to reproduce the behavior:

1. Start a backup in one profile
2. Go to another profile with a different repository, and try to start a backup
   Result: Backup button is greyed out, even though you can run multiple backups at a time from the command line

Desktop (please complete the following information):

• OS: Ubuntu 20.04
• Vorta version: 0.7.1
• Borg version: 1.1.11

Vorta and Borg versions can be found in Main Window > Misc Tab

Additional context
Must fix for #103

[m3nu]

That's correct. There can only be one Borg process at a time. The logging and other things are all set up on this assumption. To change it:

• The Borg lock needs to work at the profile level, not globally. (The same repo can be used in multipe profiles so this case needs handling too)
• How to deal with logs and status updates, when multiple processes are running?
• ...

We can keep this issue open, but it's a bigger project for sure.

[samuel-w]

For checking borg lock we can check if lock.exclusive exists in the repo url. Not sure about remote.

Status updates we could separate by profile when multiple are running, so that you could only see the current profile's status.
Logs are thread safe already, https://docs.python.org/3/library/logging.html#thread-safety. We could append the repo url for clarity.

[samuel-w]

As an alternative, we could implement multiple backups as a queue. It would be easier, just run the next in queue after the current ends.