MAGN-4378: Only 1 library category is open at a given time

# Description:
e.g. opening geometry collapses core
# Solution:
New Style for Expander.
# Reviewers
@Benglin, please take a look.

Link to YouTrack:
[MAGN-4378 Only 1 library category is open at a given time](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4378)
Oops, long code. Let's put ```<Binding>``` in a separate tag, I'll check back later.