feature: support for column auto-resize

It would be nice if Postico would support column auto-resize when double clickin the right edge of a header column.

## What did you do?

I double clicked on the right side of a column header (where the cursor is in resize mode).

## What did you expect to happen?

I expected that the clicked column auto-resizes itself to fit its content in one line.

## What actually happened?

Nothing happened, Portico doesn't support this currently.

## What software versions are you using?

Postico version: Version 1.4.3 (3766)
macOS version: MacOS Mojave 10.14 (18A391)

Start by locating the table column-header resize handling and the event path for a double click on the resize edge. Check how current resizing determines column width, then add coverage if relevant tests exist. Done means a double click fits the selected column's content on one line without changing unrelated columns.

Written by the indexing model from the issue text.