MAGN-4369: icons loading(Stream to MemoryStream)

# Preface
Once it was discussed [here](https://github.com/DynamoDS/Dynamo/pull/2234/files#r16885347), that it would be great to find opportunity of removing `MemoryStream`.
I found another solution with using `BitmapSource`, but I can not say, that it's better.
This solution was found in [this article](http://stackoverflow.com/questions/94456/load-a-wpf-bitmapimage-from-a-system-drawing-bitmap).
All in all, we need to choose one.
# Reviewers
@Benglin,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-4369 DUI: icons loading(Stream to MemoryStream)](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4369)
Rename this to ```BitmapSource bitmapSource```, and also move it to within ```try``` block. We can add to ```cachedIcons``` and return from ```try``` block directly.