[Enhancement] Provide default GC settings to prevent unreasonable memory usage

[JanecekPetr]

Description

On my Mac (with 32 GB of available RAM) a long-running muCommander often takes around 6 GB of memory, according to the Activity Monitor app. I seem to remember that the default Xmx in Java is 1/4 of all the RAM, and muCommander sometimes hits that.

I can (and just did) set my own Xmx in muCommander.cfg, however it would be nice if there was a reasonable default, say -Xmx1G or -Xmx2G? Or maybe we could keep the default, but set -XX:G1PeriodicGCInterval instead to force muCommander to give some of the claimed memory back if idle.