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.
Thanks for merging this, I did not know this is conflicting with ```selectedClassProspectiveIndex``` (which makes the name not so... appropriate now).