Feature branch for session management (ThreadLocal Pattern)
SessionManagement based on Request-Session;

The Environment class is no longer a Singleton. The UI-Class now
provides a method to access the current session. Abstract UI classes
for NavigationView and Popover with prefix „Sessioned“ were added to
avoid long-winded getSession method calls.
Merge branch 'develop' into sessions
Merge branch 'sessions' into develop