Make writing stats word calculation more intuitive

[vkbo]

The session log file actually logs the total word count for each entry, not the change in words. The writing stats dialog calculates the difference and list the change in word count per row. However, when the negative word counts are filtered out, the difference displayed in the next positive word count row is calculated from the difference from before the skipped row, so the total count may no longer actually be positive.

Example:

Day 1: +2000 words
Day 2: -500 words
Day 3: +1500 words

Filtering out the negative would then show:

Day 1: +2000 words
Day 3: +1000 words

With this change, it instead shows:

Day 1: +2000 words
Day 3: +1500 words

This makes more sense, but it also means that the sum of rows no longer matches the total word count if negative counts are excluded. But that is sort of expected, so not really an issue.