Add option to view all keyboard shortcuts available in the app

## Prerequisites
- [X] I have searched open and closed issues for duplicates.

## Feature
**Is your feature request related to a problem? Please describe.**

I don't know if there's a faster way to do something with the keyboard instead of the mouse because I don't know the keyboard shortcuts available in the app.

**Describe the solution you'd like**

An additional entry under the app's settings called "Keyboard Shortcuts". Selecting it should open a window that displays all the keyboard shortcuts available in the app. We should be able to continue working in code while the keyboard shortcuts window is still open.

**Existing work**

When you press the super key, the elementaryOS shows a list of keyboard shortcuts available. Shortcuts are grouped in sections
that describe the context of their usage.

**Additional context**
https://elementaryos.stackexchange.com/questions/17256/what-keyboard-shortcuts-are-available-in-code