Add RedDeer source feature to the site

This commit enables source plugins and source feature
and also adds the source feature to the generated update site.
Also, it updates tycho to 0.16 and fixes a typo in
a build.properties file.
Initial commit of requirements plugin.
This commit includes implementation of CleanWorkspace requirement with
tests.
Move to Tycho 0.17 and use the improvements in source feature plugin

The source feature no longer has the ugly name but I didn't like
the default name which is <featureName> Developer Resources,
so I created a custom feature.properties file.
Added logic for CompositeWidgetLocator