Add the ability to export custom icons

### Checks

- [x] I have read the Wiki, searched the open issues, and still think this is a new feature.

### Explain the problem clearly and succinctly:

Add the ability to export custom icons present in a database.

### Describe the solution you'd like:

an additional button besides the trashcan to export the icons

### Describe alternatives you've considered:

_No response_

### Additional context:

I made a new databse in keepassDX, but have icons in an older database(from when i used keepass for windows), and it would be nice if to have them exported and imported into the new db.

Start at the database's custom-icon management screen and the existing trashcan action, then trace how icons are stored and how database import is handled. Add an export path for those icons and verify that they can be imported into the new database; the issue names no files or tests, so the relevant entry points must first be located.

Written by the indexing model from the issue text.