PWD: As an end-user, I want to interact with the opened sub-app

When a sub-app is open, the user should be able to interact with it in the same way for every sub-app.

• When clicking within the borders of the sub-app it should get "focus" and by that, be shown above other intersecting sub-apps.
• The sub-app should be "draggable" by clicking (mouse-down) on the top bar and dragging the mouse.
• The user should be able to close a sub-app by clicking on an element within the sub-apps window.