[Enhancement] Allow for extensions without changing core code

[schams-net]

Problem

The documentation (file README.md) reads:

You can easily add E-mail verification, Google authentication... in your version of password manager. Put your implementation inside src/rest/check.php, which is used for login authentication.

This encourages users to edit a core file, which -- when application is updated -- is overwritten from the file from the repository.

Suggested Solution

Change application to allow users to extend functionality, without the need to chance core files. For example by introducing a Signals/Slots concept.

[zeruniverse]

Is this done?

[BenjaminHae]

It's part of the complete rewrite. But not implemented yet.

[zeruniverse]

For new feature, goto #248