DUI: NoMatchFound Button added.

So, it's done. I removed commented code, added `NoMatchesFoundButton` to `LibrarySearchView` and created appropriate event, when user press on it.
This event clears `SearchText` in ViewModel, as result search textbox clears as well.
:innocent:
Reviewers
@Benglin,@vmoyseenko, please take a look.

Tasks on YouTrack:
[MAGN-4806](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4806) DUI: Present "No matches found" label if nothing is found
I see, well the general practice is to have a ```null``` check if the data can turn out to be a ```null```, that means the code is tolerant to these scenarios. But in this case, if the ```DataContext``` is not ```SearchViewModel```, we should just let it crash (so that the problem is highlighted) rather than silently ignore it and move on. The side-effect of that (```searchViewModel.SearchText``` is **not** set to empty string) may not be so obvious to the tester. Does that make sense?