Improve the Plotwidget

Summary:

• newPlot -> addPlot
• deletePlot -> removePlot
• update is now called when appendPoint is called and is private.
• QStringList plots () returns list of plots.
• setMaximumTemperature to set the Y cap
• setMaximumPoints to set how much data to hold
• Solve memory leak (by holding only so much data per point)
• No longer start with any point. (this frees clients to add them based on the device)
• Adjust test gui for these changes.

Reviewers: Atelier: AtCore, tcanabrava

Reviewed By: Atelier: AtCore, tcanabrava

Subscribers: tcanabrava, Atelier: AtCore

Tags: Atelier: AtCore

Differential Revision: https://phabricator.kde.org/D13375