Plot screenshot/save action do not include the colorbar

As it is only the plotting area is exported.

With `QWidget.grab()` it is possible to take a screenshot of all the widget content.
This is already done here: https://gitlab.esrf.fr/levantin/txs/-/blob/master/txs/app/actions.py?ref_type=heads#L9

Still have to decide whether this replaces the current actions behavior or can be set toggle through a method (and which one is the default)