Use drop-down lists for Java Settings to allow for previous entry selection

Currently the Java Settings are simple text fields and no history is stored aside from the configured value.

As a user/tester of snapshots it would be helpful to be able to select previously used values from a drop-down list for easy switching between various JVMs and JVM arguments. The history could be trimmed to the last 8-10 values without hurting useability.

Thanks!