Allow users to specify custom java executable path

These changes allow users to specify a custom path to the java executable in the advanced options dialog in case they need to use a different java executable than the one the launcher is started with or they need to use some sort of wrapper script.

One use case for this is running java through a wrapper script that starts it with optirun on Linux systems that use bumblebee to provide Nvidia-accelerated graphics on laptops using Optimus.

Signed-off-by: Lee T. Starnes <lstarnes1024@gmail.com>