Restore state when window reopen

Closed the Postico window with lots of tabs open, to upgrade version of the app. When app was reopen, all open tabs have disappear.

## What did you do?

- Open Postico app
- Open any connection
- Click `Ctrl + T` to open few tabs
- In each tab open some table, make request, etc (like you do in your work, opening 10 tabs at the same time)
- Now close the window of the Postico (connection window and then window of the app)
- Now run the Postico app one more time and open the same connection. You will see that window has its of tables, and all you open tabs from the last session have disappear

## What did you expect to happen?

- When I open the same connection, it should restore the previous session. Like it's in all programs, including MySQL Workbench

## What actually happened?

- When the connection open, it's new session started and all open tabs from the last session disappear

## What software versions are you using?

Postico version: 1.14.12
macOS version: Catalina (10.15.4)

PostgreSQL version: 9.6.12

Start by reproducing the reported steps in the Postico app, using the connection window and several tabs before closing and reopening it. Trace how the app opens the same connection after relaunch. Done means the previous connection window restores its open tabs and their session state instead of starting a new session.

Written by the indexing model from the issue text.