Explicitly delete and replace self.plot when update_plot is called

[dgdekoning]

Fixes #286, possibly.

By replacing the self.plot object in-place in the layout, plotting changed values and finally calling updateGeometry on the widget that owns the layout it should avoid the redraw (and scaling?) issue for macOS devices.

[coveralls]

Coverage decreased (-0.3%) to 51.342% when pulling aaf5e3c on force-plot-replace-mac into 2dff150 on master.

[dgdekoning]

@nheeren, @cmutel, can either of you test this branch to see if it resolves the issue?

On windows it seems no functionality is lost, but I'm unsure if replacing plots in-place actually resolves the scaling issue.

[cmutel]

It is better (there is no shrinkage), but there are sometimes still artifacts from previous plots:

Maybe just need to clear the figure first?

[dgdekoning]

Weeeeeird, I'll add a 'clear plot' call before the deleteLater call.