For some cases switching between classes doesn't change context of StandardPanel

#### Purpose

Context after switch between classes should be changed.

#### Modifications

`currentIndex` is removed. Used `selectedClassProspectiveIndex` instead. Main fix is in using of `TranslateSelectionIndex` to find correct index.

#### Additional references

More information how to reproduce bug you can find here: [MAGN-5230](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5230).

#### Reviewers

@aosyatnik, @Benglin, please, take a look.
I wanted to ask this to be changed in a previous pull request [but did not](https://github.com/Benglin/Dynamo/pull/140#discussion-diff-19388208), so here I am asking for this change:

```cs
if (selectedClassProspectiveIndex == translatedIndex)
    return;

selectedClassProspectiveIndex = translatedIndex;
```