Support for adding/editing secret entries

[npalumbo]

Currently, the app only allows to view the secrets within an existing Keepass DB.

The scope of this issue should allow:

• Allow to add/edit entries in an existing keepass DB file, saving to the original file location.
• Should support both groups and plain entries.

Tasks:

• [x] Implement in memory operations (https://github.com/npalumbo/keepassui/pull/24) .
• [x] Add a delete button on the UI (https://github.com/npalumbo/keepassui/pull/25).
• [x] Add a new button to add group and entries on the UI (https://github.com/npalumbo/keepassui/pull/27).
• [x] Add a modify button to edit group and entries on the UI (https://github.com/npalumbo/keepassui/pull/28).
• [x] Add a save button to store the changes (https://github.com/npalumbo/keepassui/pull/26).