Expose MdiSubWindow hide() & show() to python. Re #4282.

This makes calling show & hide work from python instead of leaving the
'wrapper' window there.
HOWEVER, I'm not sure we should be overriding these methods - they are not
virtual methods (neither is close()). setVisible() - which these two call
- IS virtual, but when I tried to implement that instead it completely
messed up the appearance of the instrument window and the custom interfaces
for some reason.

Code/Mantid/MantidPlot/src/qti.sip

@@ -58,6 +58,8 @@ public:
   Folder* folder();
 
   void askOnCloseEvent(bool)/PyName=confirmClose/;
+  void show();
+  void hide();
   bool close();
 
 private: