[NuGet] Provide simplified API to be used when installing components.

Moved logic for installing NuGet packages and getting the packages
installed in a project from ComponentReferencingProjectAdaptor in the
Xamarin.Ide project (md-addins) into the NuGet addin.
[NuGet] Provide package add/remove events for components integration.

Extend the IPackageManagementProjectOperations interface so it has
events for a package being added or removed. Note that these events
are only fired if a package is selected in the Add Packages dialog
and added, or if the package is selected in the Solutions window and
removed. Events will not fire for any package dependencies that are
added or removed. This should be OK for components since the events
are being used as a way to detect if the NuGet packages in a project
have changed and not if a particular NuGet package was added or
removed.
[NuGet] Fix Packages folder not showing items during Component install

When a Xamarin Component that has NuGet package dependencies was being
installed the Packages folder would not show any packages until the
installation finished. This was caused by a null reference exception
in the Packages node builder since it was assuming that a full
IPackage was available in the pending package install actions. However
for a Xamarin Component installation only the package id and package
version are available. The install package action allows either the
IPackage to be specified or the package id and version. To fix this
the Packages folder node builder asks the install package action for
the package id and version, which is taken from either the IPackage
or the properties on the action itself.
[NuGet] Components integration - package install waits for completion.

NuGet packages are still installed using the background dispatcher but
the InstallPackages method waits for the NuGet packages install to
complete. This makes the API more consistent with what Visual Studio
provides. Also it fixes the problem where a Xamarin component may
have multiple NuGet packages that need to be installed and after
installing the first NuGet package the Xamarin Component would
previously show an error since it was missing packages that were
currently in the process of being installed. Now the installation is
finished before the Xamarin components UI is updated.

Also make the components API thread safe even though it is called on
the UI thread by the components addin.
Updated reference to md-addins
Updated reference to md-addins
Updated reference to md-addins
Updated reference to md-addins
Updated reference to md-addins
[NuGet] Add support for new Xamarin target frameworks.

Xamarin target frameworks:

    Xamarin.iOS
    Xamarin.Mac
    Xamarin.PlayStation3
    Xamarin.PlayStation4
    Xamarin.PlayStationVita
    Xamarin.Xbox360
    Xamarin.XboxOne

Source code changes:
    Commit: https://github.com/mrward/nuget/commit/0225161b94a87e720d2d95d43a3b08156c9c36cf
Updated reference to md-addins
[Ide] Open File dialog error when selecting text encoding
Problem was that some code used "MonoDevelop.Ide.SelectEncodingsDialog.ConversionEncodings" and some "MonoDevelop.Projects.Text.ConversionEncodings" property... So I removed "MonoDevelop.Ide.SelectEncodingsDialog.ConversionEncodings"
Bump md-addins
[Core] Improved msbuild build host crash handling

If the msbuild host crashes, dispose the local proxy, so that the next
msbuild operation will create a new host.

Partially fixes bug #22276.
bumped md-addins
bumped md-addins
bumped md-addins
Bump to fix issues after the lib directory moved
Updated reference to md-addins
[Core] Fix project reference metadata loading bug

Properly read metadata for project references that reference other
projects. This fixes bug #22044.

In the msbuild host, only reload the project that is going to be built,
not all referenced projects.