Reverts #243

I tested the test specification in #243 before merging the Pull Request and after Merging it an found no difference in the outlined behavior:

to reproduce the issue:

Open the MVVM example, select the "Aero" theme (makes the steps in the ui easier)
Open 2 documents. the 2 documents are displayed in 2 distinct tabs (correct)
Change documents by clicking on the tab header, outside the text label (with aero theme, click on the spline on the left part of > the tab). The displayed tab changes (correct), the lower toolwindow (display of the document properties) does not change (incorrect).
The lower toolwindow is bound on the ActiveDocument (defined by the "IsActive" flag), the displayed tab is based on the "IsSelected" flag.

So, I am sorry (should have test this more carefully - dahh ... - ) but I am gone have to revert this :-(