Since there was so much debate regarding the unit testing of the visiblity presets, i had another look at it.
The old version was almost untestable because all functionalities were mushed together into a single class.
CURA-5734
several instances need to be created.
- In the ThreeMFWorkspaceReader we need to create some temporal
instances of Preferences that makes it not singleton anymore.
- The current preferences are kept in the Application class and so all
the calls to the preferences are changed to get the preferences from
Application.
- The method getInstance in Preferences is kept as deprecated since some
external plugins.
