Close tooltip after click. Two more small fixes

#### Purpose

1. Close tooltip after click on member. Tooltip should be hown on hovering on other member.
2. Make hover background for all class and member the same `#404040`. Check it on `LibraryView` and `LibrarySearchView`.
3. Top result should be visible on top of search results when search text is changed.

#### Modifications

Changed XAML. Added new event `SearchViewModel.SearchChanged` to bring top result on top of search results.

#### Additional references

[MAGN-5605](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5605).
[MAGN-5606](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5606).
[MAGN-5607](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5607).

#### Reviewers

@Benglin, these changes are small. So I united them into one PR. Please, take a look.
Please use `EventArgs.Empty` here instead of creating a new one.