GUI - add graph visualisation popup

[shawnlaffan]

It would be handy to visualise some of the array lists and hash outputs with numeric keys. These can be simple line graphs.

Hopefully there is a Gtk2 graphing widget already. If not then we will need to build one.

To do:

• [x] Decide if we want multiple popups or for the contents to change on hover. Prob the latter.
• [ ] Support multiple plots in each popup (e.g. related indices). Need index metadata to help this.
• [ ] User can select colours.
• [ ] Rescale axes?
• [ ] Change number formats for axis labels
• [ ] Log scaling for axes
• [ ] Sort out the fontconfig issue under windows. Currently it is being manually copied across between versions.