Make DefaultPlatform.open() return quickly, always

[ctrueden]

On MacOSX and Windows, open(url) will return immediately. On Linux, open(url) will immediately only if there was already a running web browser instance.

We might need to do something as hacky as trying to execute the launcher, waiting for 50ms and checking whether Process.exitValue() indicates that the executable was not found (it throws an IllegalThreadStateException if the process has not yet terminated). Probably it would be a good idea to wait a little bit longer to see whether it still wants to terminate with an error code, just in case the machine was busy.

This change is needed to make things behave consistently between platforms.

Migrated-From: http://trac.imagej.net/ticket/1419