Added a function to create a startpage.
Added a function to remove the startpage.
added a function to check if startpage is active.
Added functionality to create a new tab.
Added "Draw in new Tab" Button and moved the drawing from the
actionlistener to a method.
Added "Draw in new Tab" Button to InclusionResultDialog and moved the
drawing from the actionlistener to a method.
Added "Draw in new Tab" Button to OpenProblemDialog and moved the
drawing from the actionlistener to a method.