Add individual export

These commits adapt the export function so it better fits multi trace display. There is an export button for each trace.

Exporting behaviors:

When exporting a trace in command line:

• This behavior was not changed It always export to .svg on the entire y range and on the x range defined by the -t or -T command line options When several traces are put in the command line, only the first one is exported and an error message occurs.

When exporting a trace with the button:

• Previously .svg exported the trace on the entire y range and on the x range set by the -t or -T command line options Now the trace is exported on the entire y range and on the visible x range

  These other behaviors have not changed

• If it is exported as .txt a box opens to choose which variable to export

• Otherwise an image of the current view is saved

A ToolTip was added to Export, is was also added to other individual buttons at the same time. Both zoom icon were deleted because they were unused and I didn't want to make a MR just for that