Add ability to generate QR code of username and password fields

When trying to quickly authenticate on another device that is either not setup with your database and or KeePass DB reading application because it is new or not your own, it can be quite a hassle. Typing over credentials or installing the application and getting the DB can be annoying when you have to be quick. Most operating systems do have built in or easy ways to scan a QR code however, which could solve this problem.

By adding a 'generate QR' button next to the existing copy value button, a simple plaintext QR code is generated that contains that fields' value. This can then be used to scan on the other device and does not require any software or files(such as your own DB) to be available. Both Android and iOS have this functionality built-in. On Windows, MacOS and others, you might still need to install a scanning application, but this still alleviates the need to share the DB file.

This experience could be further improved/expanded upon by software on the receiving/scanning end of this solution. But the key is that it is not required.

Similar functionality is provided by some plugins for the desktop app such as: https://github.com/JanisEst/KeePassQRCodeView

Start by locating the existing copy-value button for username and password fields in the Android app and trace its surrounding UI and actions. Determine where a QR-generation action would fit, then verify that the generated QR content is the field value as plaintext and that both credential field types are covered.

Written by the indexing model from the issue text.