Remove all hardwired paths from launch configurations

When generating launch configurations, it is necessary to include components from inside of the currently installed org.codehaus.groovy bundle (such as the groovy-starter.conf file). Currently, we use hard wired fully qualified paths for this. However, is bad for at least two reasons:

1. these launch configurations cannot be shared between users
2. after an upgrade of groovy-eclipse, the files pointed to by the launch config may no longer be there.

The solution is to create a launch configuration variable that points to the currently used org.codehaus.groovy bundle in the installation folder and use that variable instead of hard-wired paths. I'm not entirely sure if it is possible to create custom launch config variables and if it isn't then we will need to look at some other solution.