Perspectives: Stack & BP view can appear tabbed unintentionally

Steps to reproduce:

• start QC with clean settings
• create a project from template
• switch to Debug view, switch to QmlProfiler and back to Debugger inside the debugger tool bar
• switch to Help view
• switch to Projects view
• start debugging (F5 / main 'Start debugging' button,..)

Current result:

• stack widget and bp widget appear tabbed instead of beside each other