Export session info data

[Boruch]

From the system tray menu, the menu item "session info" allows for limited export of data. This ticket suggests expanding that functionality.

1] Currently, it seems that only the tab "Graphs" offers this, in the form of saving a graph as a .png file. However, for that particular functionality to be useful, the graph needs more environment information, including a session identifier, date, time, endpoint machine IDs, and interface ID.

1.1] Anyone who wanted to seriously use the graph data would want it available in some text form, eg. csv.

2] More generally, each of the other five tabs don't seem to have a way to copy or export or otherwise save the data that they present. My failed attempts included:

2.1] selecting a region with a mouse

2.2] right clicking on a region and selecting either 'copy' or 'select all' and then C-c.

[Antoine Martin]

This ticket suggests expanding that functionality.

The recommended way of exporting session information is through the bug report tool and with xpra info.
Both will capture very detailed information, including pretty much everything shown on the session-info dialog and more.

The graphing code needs to be re-written for python3 (#1568), so maybe we can improve it then.