gui: add save histogram image

[gadfort]

Changes:

• reorganized the ChartsWidget to ensure the histogramwidget owns all the display data to allow for easier integration with the save image code.
• added a save option to the mouse menu
• added access from tcl to save the image via save_histogram_image
• added hold timing to to bar chart

[github-actions]

clang-tidy made some suggestions

[maliberty]

The default size is quite small and when zoomed in shows the labels don't fit

[gadfort]

@maliberty the default just takes the size of the widget from the GUI, I can update it enforce the height is == width when not specified. (unfortunately this size if dependent on the state of your gui, so it's a bit difficult to replicate). This mirrors the behavior of the clock widget, but maybe it's better to select a default like (500x500 or 1000x1000) which tend to generate reasonable images (and then ignore the widget size altogether).

[maliberty]

Thanks. It works fine aside from the default size. I think have some constant like you suggest is best.