DolphinWX: Get rid of wxGrid-based casts in the debugger.

This technically also fixes a memory leak in WatchView.cpp, because the table setting was done such that the grid wouldn't take ownership of the table, which means said table wouldn't be deleted in the grid's destructor.

Source/Core/DolphinWX/Debugger/WatchWindow.h

@@ -34,8 +34,10 @@ public:
 	void LoadAll();
 
 private:
+	void CreateGUIControls();
+
 	wxAuiManager m_mgr;
 
+	// Owned by wx. Deleted implicitly upon destruction.
 	CWatchView* m_GPRGridView;
-	void CreateGUIControls();
 };