Fixes for running Visomics from the build tree

The application should no longer complain about not being able to find
GetRExternalPackages.R at startup.  It also now looks in the source
directory's "scripts" dir for custom analyses.

These changes only apply if you're running Visomics from a build tree
(not an install tree)
Merge branch 'fix_build_tree_Visomics'

* fix_build_tree_Visomics:
  Fixes for running Visomics from the build tree