- relocated the code for closing a DockItem into an own function "Close()"
- changed the "X" button in the window title to invoke that function instead of doing everything hardcoded in its own delegate
- cleaned up the right-click window title context menu:
      * hid the "hide" and "autohide" menu entries, as we do not use this concept currently
      * added a "close" context menu entry and invoked the function mentioned above