[FEATURE] Implement handling of instantiated services

[dehesselle]

Is your feature request related to a problem? Please describe.
systemd-manager-tui can't handle instantiated services.

Describe the solution you'd like
In order to start a service myservice@.service you'd issue the command systemctl start myservice@instancename. systemd-manager-tui could check if a service has @ in its filename, prompt the user for a name and then issue the start command accordingly. The same applies stop, enable, disable etc.

Describe alternatives you've considered
None. Instantiated services are a core feature of systemd.

Additional context
I've tested some other systemd TUIs and none of them cover instantiated services. This is the only part I'm missing, overall I like systemd-manager-tui to become my daily driver.

[matheus-git]

Try the shortcut 'f', it lists ALL units.

[dehesselle]

Now try to start one.