Added our version of PXListView to project.
Revamped version of UnifiedDisplayView class.

Basically : UnifiedDisplayView is a hacked copy of MessageListView, but the articleList instance variable is now a PXListView (subclass of NSScrollView) instead of a MessageListView (subclass of NSTableView).
ArticleCellView's are cells, each containing a WebView.
Better handling of layout switching
Better handling of filter bar hiding.
Hackish way to force Unified layout first display (at application launch) to be OK.
Safety measure for PXListView
Use ArticleController as a controller for certain functionalities…
Activate Article menu items for Unified layout.
Fix to over-scroll control
Adapt layout names as discussed in issue #176 for English, French and German translations.
Clarified and commented code for handling Report (horizontal) and Condensed (vertical) layouts
Merge pull request #191 from barijaona/unified-revamped

Completely new handling of Unified layout.