Show next 5 members when last member is partially visible

#### Purpose

`LibrarySearchView` should be scrolled if user goes to the most bottom member. Should be shown next 5 members.

#### Modifications

Proposed new function which defines coordinates of currently highlighted member and calculates coordinates of last visible item. Using `ListviewItem.BringIntoView()` item is made visible to user.

#### Additional references

[MAGN-5725](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5725) DUI: Down arrow button on latest visible member should scroll down for next 5 members.

This PR implements scrolling down. Scrolling up can be implemented as new PR or as part of it PR. Anyway this PR should be merged after https://github.com/Benglin/Dynamo/pull/251.

#### Reviewers

@Benglin, please, take a look.