Saving gputop-ui timeline graph info

[eero-t]

gputop-ui timelines are very small, so one cannot see smaller changes, or easily compare them.

It would help if one could save data visible in the current graph to a CSV file, preferably in same format as gputop-wrapper, so that same post-processing tools can be used for it.

This way one can start sampling, start test-case and stop sampling, then save the data.

GUI-wise there's free space above timelines for the "Save button". If ImGui doesn't contain file selector, gputop-ui could just save the files with the pre-determined name to progam's startup directory with the running index added to file name, and it's path/name output to terminal or dialog box.