Add a shortcut editor example

As a Qt API user,

I would like to have a shortcut editor example,

so that I can use that as a reference when implementing a shortcut editor in a Qt application.

Here is the list of features I have implemented in my example: https://github.com/lpapp/examples/tree/main/qt-shortcut-editor

This is a bit overwhelming list, so I would like to suggest to trim the features provided by the example to this basic subset:

1. Tree view of the shortcuts by context (2 contexts for simplicity).
2. Changing shortcuts (Assign, Change, Remove).

I am not sure about save and load. Yes, any shortcut editor needs it, but it seems like just generic serialisation and deserialisation we already have examples for. Same thing with other features, like undo, etc, these do not need to be in a very basic example.

The aim of the example would be to demonstrate the various features of QKeySequence and QKeySequence edit including their use and features in the Qt Model/View framework.