Track PB's on a per-run basis

[MichaelJBerk]

From @jseakle #102

PBs seem to be tracked per-split rather than per-run. This is totally unlike how all major timers, and not very useful. Especially combined with the fact that undoing a split doesn't undo the change to your PB, this means that the only way to recover from pressing the split button twice by accident is to quit without saving, which is terrible. I should be open a file, start a run and do some splits, then mash Z to the beginning of the current run and save, and have there be no changes to the file.

The normal thing is to keep a whole set of splits grouped together as a run, and then show the run with the lowest total time as the PB splits. Per-split PBs should also be tracked (i suppose as an optional column, but usually invisibly) and should be colored gold when they appear. Seeing a weird mishmash of my best ever splits in the PB column is essentially useless.