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.