Fix NPE when env var not found
Rename property page.
Updated UserGuide, Features, and screenshots to new DUB features.
Refactoring of some JDT copied code, created src-dt-fork/ .
First stage refactoring, copied compilers pref page from DLTK.
Fixed compiler install replacement.
Full refactoring and rewrite of compilers preference page.
Did pending doc update with missing Compilers page screenshot.