X10DT should provide a notion X10 runtime library "compatibility level"

Currently, X10 projects in a user's workspace get bound to a concrete version of the X10 runtime. This causes the projects to break under at least 2 scenarios:

• when the X10DT gets updated, potentially bringing with it a new version of the X10 runtime, or
• when the project is checked out from CVS/SVN into an X10DT installation that bundled a different X10 runtime version

To address this issue, the X10DT should provide a mechanism whereby a project can be configured to depend on a given "X10 runtime compatibility level," that implies binary compatibility with all such runtime library releases.

This is analogous to Java's compatibility levels, e.g. Java 1.4, Java 5, or Java 6, which are supported by the Eclipse JDT.