Left sidebar width should be restored when Dynamo loads

#### Purpose

Width of sidebar with `LibraryView` should be saved when Dynamo closed and restored when Dynamo started.

#### Modifications

Added property `PreferenceSettings.LeftSidebarWidth` to save / load width.

#### Additional references

[MAGN-5721](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5721) DUI: Left sidebar width should be restored when Dynamo loads

#### Reviewers

@Benglin, please, take a look.