Rechecked if IntroPart was created in OpenIntro and enforce its visibility.

I am not sure if I find the reason/solution.
But at least it seems to be a sensible mitigation. Fixed T27549.

Following observations:

• MITK Diffusion has not such problems, but they use there own application/plugin/windowadvisor. In the diffusion window advisor the IntroPart is coded to always being shown, thus no problem.
• The IntroPart is activated correctly, but afterwards in QmitkExtWorkbenchWindowAdvisor::PostWindowOpen() mitk::WorkbenchUtil::OpenEditor() is called which brings the editor up front again.
• I could not identify any code change that triggered the change in behavior of blueberry (but as I wasn't shure since when the behavior changed, the search was only very shallow).
• You can circumvent the proplem, if you check in QmitkExtWorkbenchWindowAdvisor::PostWindowOpen() after mitk::WorkbenchUtil::OpenEditor() if a IntroPart is there and force its showing (again).