Add PerspectiveButtons widget
Merge pull request #472 from anserran/perspectives

Add PerspectiveButtons widget
Default language .props file now looks like that of any other language
* i18n.props "default" key must now reference another key
* messages.props for default key must now sport proper language
* language files are now lazy-loaded at first resolution
Apply new convention to all existing language files
Fix references from other java files
Fix comments before merge
Added javadoc to newly-public constants in I18N