Changed minimum width of sidebar to include 2 classes

#### Purpose

Reduce minimum width of Library sidebar to include 2 classes instead of 3.

#### Modifications

Changed width value. Screenshot of result:
![image](https://cloud.githubusercontent.com/assets/8158551/5204303/f0003330-7596-11e4-83c1-c2c7695e5ea0.png)


#### Additional references

[MAGN-5528](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5528).

#### Reviewers

@ramramps, please, take a look.

@Benglin, FYI.