Flint: Clean up around custom plots

Closes #2051 and #2143

See merge request !3780

Remove 'here' from the hyperlink

See merge request !3788

Pi 517 518 init and tests

Closes #2799

See merge request !3782

Added helper for scans watcher

See merge request !3772

Merge branch '2813-newport-xps-cannot-read-position-with-several-axes-of-the-same-group' into 'master'

Resolve "Newport XPS: cannot read position with several axes of the same group"

Closes #2813

See merge request !3783

- Remove dedicated constant created in flint test session

- In the end this class should not be needed anymore

- This fixes small inconsistancies

- plot_scatter is not using anymore the previous API
- Add extra tests