Add export tools

[Fred Bertolus]

Add an menu entry to export a exercise perroquet file :

as template perroquet
as perroquet package

The menu entry could be in :

File > Export > Export as template...
                        Export as package...

[Fred Bertolus (fred-bertolus)]

Pseudo code for Core

def onExport():
 self.gui.AskPropertiesAdvanced()
 template = copy(self.GetExercise())

 if template.isLockAtExport():
  template.setLock(True)
 template.setTemplate(True)
 Export(template)