Allow users to specify VM arguments

Added preference for specifying VM arguments to be used by worksheet processes.
Adjusted preference page layout to accomodate for new field
Error dialog reports last output upon abnormal worksheet termination

Fix #125
Removed string interpolation Re #125
Switched over to DisplayThread.asyncExec Re #125
Merge pull request #131 from nadavwr/vmargs-#125

Allow users to specify VM arguments