Added saving and loading for edit mode

MainWindow - Added functionality to save and load when selected from
file.

GameSystem - Added a saveSceneManager and a loadSceneManager function.

SceneManager
- Added a saveToFile that takes in a String of the file directory
followed directly by the file name and attempts to save the current
SceneManager details (Scene Graph and Item List) in XML format to file.
- Added a loadFromFile that takes in a String containing the file
directory followed immediately by the file name and attempts to load the
Scene Graph and Item List from the file given.

SceneTableModel
- Added a function that lets the caller set a new Scene Graph in the
SceneTableModel (used for loading from file)

ItemTableModel
- Added a function that lets the caller set a new Item List in the
ItemTableModel (used for loading from file)
- Added a column that tells the name of the scene where the item drops

Connected the XStream libraries for file I/O
- Tyrone