Allow Reflexil integration

[sailro]

When writing an ILSpy plugin (in my case Reflexil), it is very useful to:

• know when the selected node(s) is changed, so that the plugin can react in sync with the user selection. You already consume SelectionChanged from the TreeView, so we can easily forward this with a SelectionChanged event directly exposed by the MainWindow.
• use the AssemblyReferenceTreeNode node and its AssemblyNameReference object. In my case, Reflexil is able to deal with such objects.
• Hot replace an assembly from stream so that ILSpy and Reflexil can exchange data, without the need to share the same Mono.Cecil implementation.

Regards
S.L.