QMdiArea: class updated
QMdiArea: class updated

- added methods tabsClosable and setTabsClosable
- added methods tabsMovable and setTabsMovable
- method setActiveSubWindow changed to accept NIL as parameter 
- parameters checking
- formatting

Modified:
source/QtWidgets/QMdiArea.prg
marcosgambeta/Qt5xHb,marcosgambeta/Qt5xHb