Save plotted data into files

[zhujun98]

Dmitry requested again today to add the functionality to save the plotted data into files. One way to do this is to have an option in the right-click manual of each widget. But for now, I am reluctant to touch the right-click menu which is implemented in pyqtgraph.

I came up with another solution: we could have a button in each window to save the data in all the plot widgets. The data can be saved into a single file or different files.