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.
I suggest removing this comment here and move it to `OnExpanderButtonMouseLeftButtonUp` below. And then do this here:

```csharp
// See OnExpanderButtonMouseLeftButtonUp for details.
private bool ignoreMouseEnter;
```

```csharp
// Clicking on a member node results in a node being placed on the canvas.
// Another mouse-move event will be sent right after this left-button-up, 
// which brings up the tool-tip. This isn't desirable, so setting a flag 
// here to ignore the immediate mouse-move event.
private void OnExpanderButtonMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
    // Bla bla bla...
}
```