Disable file migration for 0.7.0

This pull request addresses the following defect:

[MAGN-2381 Disable auto migration and warn users if they are trying to open older version file.](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-2381)

It also implements a new ```GenericTaskDialog``` for use in displaying the error message. In the near future, ```CrashPrompt``` will be replaced by this dialog.

##### A note about dummy nodes
A dummy node can be seen in the following scenarios:
1. When a migration method decides to make one
2. When a type of a node cannot be resolved
3. When a function cannot be resolved in ```DSFunction``` or ```DSVarArgFunction```

Item #1 can no longer happen after this change, but the other two are still pretty much valid use cases. So please do not be alarmed if dummy nodes show up on canvas.