Do not remove grouping information for custom nodes, imported libraries

#### Purpose

Do not remove grouping information for custom nodes, imported libraries

#### Modification

Made `if`s near call of `ProcessNodeCategory`. Grouping information is removed just for `Regular` nodes.

#### Additional references

[MAGN-5200](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5200).

#### Reviewers

@aosyatnik, @Benglin, please, take a look.
Please make this short and sweet:

```cs
// Rename category (except for custom nodes, imported libraries)
string category = ProcessNodeCategory(function.Category, ref group);
if (functionGroup.ElementType != ElementType.Regular)
    category = function.Category;
```