Fix a showstopper as CBN fails to show previews and geometries

##### Background
This submission fixes the following show-stopper defect (tracked internally):

[MAGN-3236 Geometry doesn't appear from CBN](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-3236)

The following changes were made for this fix:

* ```NodeModel.AstIdentifierBase``` is now made ```public``` and ```virtual``` (for CBN)
* Identical property ```NodeModel.VariableToPreview``` is replaced by ```AstIdentifierBase```
* Fixed an incorrect condition in ```NodeModel.PrintValue``` (for info bubble display)
* Test cases are updated to refer to ```AstIdentifierBase``` instead of ```VariableToPreview```

##### Preview
This is how the fix worked on my machine:
![preview-geometry-issues-with-cbn](https://cloud.githubusercontent.com/assets/5086849/2860810/e4a1d5e6-d1d3-11e3-97d4-d4f8a06ae61b.png)

##### Testing Scenarios
I have tested with the following cases and it appears to be working fine:

* Multiple lines with simple data types in CBN
* Multiple lines with geometries in CBN
* Modified CBN contents by renaming variables
* Modified CBN contents by changing literal values
* Modified CBN contents by changing orders of lines
* Undo and redo above modifications