[FEAT] Add Zoom in - Zoom out capabilities using "+" and "-" buttons

[NathalieC]

Prerequisites: #1487

To focus on a diagram part, or move away back to the global picture, the user should be able to use buttons to zoom in and out.

The buttons will be provided by the application that will embed the library. The library will get the signals from a "+" and a "-" buttons.

Each click on a button will change the zoom value (Ratio? Absolute value? TBD).

Linked to #731 and #732.

POC have been implemented to demonstrate this feature, see #555