MAGN-4643 DUI: Add and style Add-ons header
Refactoring according to Ben's review
"Import Library..." core logic is ready
Renamed ElementType.DLL to ElementType.CustomDll
Removing CategoryTemplateSelector.cs and "Add-ons" from the categories
Replacing StackPanel by Grid
Merge pull request #78 from vmoyseenko/DUI-ImportLibraryAddCustomNode

"Import Library..." adds nodes to CustomNodes TreeView
Merge pull request #82 from vmoyseenko/Sitrus2

Removed redundant line from csproj which left after merge
Merge pull request #75 from vmoyseenko/MAGN-4643

MAGN-4643 DUI: Add and style Add-ons header