Re-implement WindowManager to modify a model rather the WPF objects themselves

[cnapolit]

The WindowManager directly modifies WPF objects, so only the main thread can utilize WindowManager.
This makes the polling service difficult to stop, as it must invoke the main thread to perform actions on its behalf.
Since the parts of the plugin responsible for stopping the polling service are themselves on the main thread, they cannot wait for the polling service to stop since that service is often already waiting on the main thread.
Changing the WindowManager to modify a model the WPF objects derive from may allow a cleaner exit for the polling service.