Add running total to mass window

[Cameron Grout]

Currently from the 'Mass' item on the right-click menu, we can see the various transits made by ships, and the approximate mass for each jump. There does not appear to be anywhere to see the cumulative mass, meaning we need to dump the mass table into a spreadsheet to get the running total.

Would it be possible to add a running total to the mass window?

[Josh Glassmaker]

Putting on the list to do

[Josh Glassmaker]

Added feature - closing #50

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