Support for custom PB conditions

With custom key value pair support, auto splitters can provide information such as deaths and score. By allowing to modify the PB condition the LiveSplit could be used in the following scenarios:
- Runs that minimize deaths
- Runs that maximize score
- Runs that minimize the amount of items (Low%)
- Runs that maximize time (originally requested here: https://github.com/LiveSplit/LiveSplitOne/issues/695)

This is blocked for now on custom variables in general being support a bit better by livesplit-core (permanent storage in history, showing score on the timer instead of current time, ...)

Start by reviewing the custom variables work in livesplit-core and the related request in issue #695. Confirm what permanent history storage and timer display support are still missing before addressing custom PB conditions. Done should cover minimizing deaths, maximizing score or time, and minimizing item counts.

Written by the indexing model from the issue text.