"No matches found" button is shown correctly
Add-ons label is added for search results
SearchResultsToVisibilityConverter is rewritten for better security
Mark loaded CustomNode of package as ElementType.CustomNode
Merge pull request #126 from vmoyseenko/DUI-ShortenAddonsSearch

"No matches found" shouldn't appear if some AddonRootCategories element is presented
NodeSearchElements Copy() copies "ElementType" property
"ElementType" property is specified in constructor
Merge pull request #129 from vmoyseenko/DUI-CustomNodeToAddons

CustomNodeSearchElement during Dynamo loading process should be marked as "ElementType.CustomNode"