Add file ops menu logic about k4stem HOT 1 CLOSED

Implement the following logic involving the File menu

1. The app starts without a file specified
2. File > New creates an empty model and sets a "dirty" flag which remains set until saved
3. File > Open builds a model from a file
   4, File > Save should only be enabled when there is a file selected and the dirty flag is set
4. File > Save As should only be enabled when there is a file selected and the dirty flag is set
5. File > Close clears the model, flag, and file
6. File > Exit should check the dirty flag and prompt before closing

The Save operations should clear the dirty flag.

Disable Open Recent until the Preferences capability is added. (This will be a small preferences text file saved to the users hard drive.)

from k4stem.