Export, plugin archive install, and herald cache race condition fixes (#82)

* WIP: Grid export and plugin extraction

* Tweak export process

* Check for recursion of the same assembly load.

* Individual screens aren't always primary?

* Wait for cache to be writable

* Export selection only.

* Update built xml docs

* Ignore invalid archives.

* Need to ensure task is started.

ObservatoryCore/UI/Views/CoreView.axaml

@@ -73,6 +73,14 @@
           Cursor="Hand">
           Update Available
         </Button>
+		<Button
+		  Name="export"
+		  Margin="10"
+		  FontSize="15"
+		  Command="{Binding ExportGrid}"
+		  Content="Export">
+		  Export
+		</Button>
         <Button 
           Name="ToggleMonitor" 
           Margin="10"