Add "select all option" in import/export server window

[dblugeon]

Hello Pgadmin team, Describe the solution you'd like

In Import/Export Servers window, step "Database Servers", Add a button "select all databases".

Describe alternatives you've considered

There is already the python script /path/to/setup.py load-servers/dump-servers can do this, but this feature request is for users which use only the GUI.

Additional context

When there is a few server groups, the currently GUI is OK. But when there is more dozen group server, we miss a select all button in export/import step.

Regards