Added resource strings for UpdateNotifier.
Support addins with subdirectories from the online gallery.

Addin manager now copies all files and subdirectories from the extracted
NuGet package to the temporary addin installation folder.
Fix PInvoke sample.
Only show latest version of addin from online gallery.
Revert "Only show latest version of addin from online gallery."

This reverts commit f6649b343b4a45da23b2cc58c6ddc0ae4f1b44a2.
Fix: Under certain circumstances multiple releases of a package are shown in list of available packages.
Fix: List of available AddIns might show an older instead of most recent release of a package.
Now ClassBrowser tree nodes show their context menus independently.
Workaround of Delegate.Combine issues with contra-variance by introducing custom delegate lists for CollectionChanged event of models.
Showing correct path to .addin file in list of installed packages when installing an .sdaddin package or an external AddIn.
Removing error message when clicking at "Install from archive..." button.
Re-added prefiltering of available AddIn NuGet packages using IsLatestVersion, adapted unit tests.
Merge branch 'classbrowser' into newNR.
Showing target file name in action menu of MoveTypeToFile/RenameFileToMatchTypeName context actions.