vnstat: Add a Sort Order option

[gpfountz]

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Describe the problem

The vnstat widget lists the last 14 rows in ascending order. I am using this to monitor my WAN's daily usage, that is 14 rows that need to be displayed in the widget window in order to see the current and previous days. Adding the Sort Order option to the widget option menu allows me to sort the days displayed in descending order, then make the widget window smaller while still showing the most recent days.

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Describe the proposed solution

This pull request will add an option to widget's option menu labeled Sort Order with Ascending and Descending options. Ascending is the default. Making this change in the option menu keeps the actual widget window uncluttered.

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Related issue

If this pull request relates to an issue, link it here.