set activeView to null when a non-KTE::View is active
This fixes the issue that we don't have the gui client for activeView not merged into the MainWindow. Lots of code is simplified, activeView now just returns m_guiMergedView and doesn't try to do any other thing like it was doing before. ViewData::active is removed, m_guiMergedView is the source of truth now. Further improvements could be to _really ensure_ that viewspace itself doesn't do **any** view activation by itself. That means no direct showView etc from inside ViewSpaces.