SearchViewModel.SearchRootCategories collections is introduced
CategoryBuilder.RemoveEmptyCategories doesn't clear CollectionChanged eventhandler
Initially assigned eventhandler
Small reorganization
SearchViewModel.SearchRootCategories property added
Merge branch 'Sitrus2' into DUI-LibrarySearchViewVMs
OnRootCategoriesCollectionChanged is added
Reverted unneeded code change
Merge pull request #247 from vmoyseenko/DUI-LibrarySearchViewVMs

Import library doesn't add root category to library nodes tree
Merge branch 'Sitrus2' into DUI-HeaderStrip