Crash on dangling activeWindow in live preview

[vorlov-playrix]

1. Make a layout with a FrameWindow as the root
2. Switch to live preview
3. Activate a frame window (e.g. drag it)
4. Switch to visual editor
5. Switch back to live preview
6. Interaction with the window will crash an editor

[vorlov-playrix]

The problem is in Titlebar, which is an active window actually. When destroying FW, TB is destroyed as a child, but makes its parent (FW) an active window. Context in FW is not cleared yet at this point, so FW becomes active despite being already destroyed!

[vorlov-playrix]

Fixed in CEGUI