Add support for AutoOpen / child database

KeePassXC and Keepass supports automatically opening child database using AutoOpen group ( https://keepassxc.org/docs/KeePassXC_UserGuide.html#_automatic_database_opening / https://keepass.info/plugins.html#keeautoexec ).

This feature, if the application supports opening and searching several databases in memory, allows to share credentials between several people easily (without the hassle of KeeShare https://github.com/hicknhack-software/KeeShare which seems a bit less supported in Keepass* ecosystem).

Start by reviewing the linked KeePassXC and KeePass AutoOpen documentation, then determine how KeePassDX currently opens and searches databases in memory. Done means supporting automatic opening of child databases through an AutoOpen group while allowing credentials to be shared across the opened databases.

Written by the indexing model from the issue text.