Working loader&saver + bug fixes.

Enchances:
1. Loader and saver now uses it's own windows.
2. Loader and saver has some user-friendly things like title etc.

Bug fixes:
1. Saver used graphmodel not as a model.
2. After loading graph edge had unknown position.
3. Saver and loader uses *.xml extension to work with.
4. If you don't want to open file after calling FileDialog, the
following edit will not crash programm.
5. Grid after loading the file doesn't diappear if it was enabled.