Resolve "enh(ui): add signal to notify if the next activity is ready"

Description

Two signals were renamed in sight::module::ui::qt::SSequencer:

• enabledNext() -> hasNext()
• enabledPrevious() -> hasPrevious()

One signal was added, which is triggered when the next activity is enabled (all requirements are satisfied):

• nextEnabled()

Closes #954 (closed)

How to test it?

• SightCalibrator