Fixes issue #624 Add Secret changeset group to the Synchronize view

This branch adds support for a "Secret" changeset group in the Synchronize view. This fills a functionality gap that was left after completion of #614 (closed). Namely, shifting a changeset into secret phase would completely hide it from the Synchronize view, so the user would need to use the other views/dialogs to regain access to those secret changesets. With this feature branch, the user can now manage their secret changesets easily from within the Synchronize view.

A few minor cleanup commits are also included in this branch. This improves the quality/readability of some pieces of code related to this feature.

NOTE: Lately, it seems that my Eclipse is formatting the source slightly differently than it did before. As a result, some of the whitespace in the javadocs has changed slightly by the automatic formatting. This creates some noise in the diffs. Changeset e0c1cdab6cca made back in March removed some lines from the plugin\.settings\org.eclipse.jdt.core.prefs file, including some settings about indenting/aligning tag descriptions (I assume that is referring to javadoc tags), so perhaps that is related.