Track Active Workbench Element

[danielwiehl]

The Workbench should track the active workbench element (view, dialog, message box, popup) as a prerequisite for implementing selection propagation (#486). Only one element can have the focus at any one time, and only that element should contribute to the selection.

Technically, the Workbench has two active parts, one in the peripheral and one in the main areas. Views are opened in the active part of the navigation's target grid. However, as only one element can have the focus at any one time, only that part should be visually highlighted as the active part, indicating that it is the target of user interaction.

See also #486