Battery benchmark

[delt]

Simple change ideas for the battery_bench plugin:

If battery_bench.txt already exists, ASK whether to resume the benchmark, or create a new file. (battery_bench[01,02,03,etc].txt perhaps?)

Also, this might seem stupid, but SELECT also starts benchmark, and not just play. A few times i hit the wrong button, let the player run completely out of battery, then connected it and found there was no battery_bench.txt file.