AddInManager with NuGet features

This is the work done on the "new generation" of AddInManager so far. Since it might be late for 4.3, i've migrated it to SD 5. Although already described on mailing list, here are the key characteristics:

Offline (similar to original AddInManager):

* Enabling/disabling AddIns
* Opening AddIn options
* Installing AddIns directly from *.sdaddin files
* Uninstalling AddIns

New:

* Based on WPF
* Configuration of NuGet repositories in Options dialog.
* Browsing and searching in configured NuGet repositories
* Downloading, installing and updating NuGet packages with SD AddIns
* Considering the downloaded NuGet packages, when uninstalling AddIns.
* Handling dependencies defined in NuGet packages, requiring license acceptance if needed.

At least the offline part can be seen in action directly. The NuGet-related features require a repository with packages tagged with "SharpDevelopAddIn".