Feature request: double-click should expand left sidebar items

Hello! First of all, thanks for your work on this great software which is quite enjoyable to use!

Here’s a small feature request.

#### When applicable
When “Show All Databases” option is enabled for a connection.

#### What triggers the action
Double-click on a database icon in the left sidebar (where the list of databases is shown). Example of such an icon:

#### What happens now?
Database tables are shown in the main view. Nothing happens in the left sidebar.

#### What’s the desired outcome?
The list of the tables is getting shown in the left sidebar (same thing as clicking the tiny hard-to-aim arrow to the left of the icon does now).

#### How could this be done
1. Either the current behavior is replaced with a new one.
2. Or an option “Expand database on double click” is added to “Postico Preferences” so that users who are interested in the new behavior could enable it.

Thank you!

The issue names no files, tests, or entry points. First reproduce the current behavior with “Show All Databases” enabled, then double-click a database icon in the left sidebar; done means its tables expand there, with the final interaction or preference behavior decided.

Written by the indexing model from the issue text.