Load XML from EventLogReader

[bill-long]

With this change, we load the XML from the EventLogReader by calling ToXml() on the EventRecord, rather than generating our own XML from the Properties collection.

Pros:

• Full-fidelity XML instead of the partial XML we had before.

Cons:

• Loading the XML this way takes time. We do this in the background after the initial load, and the status is reflected in the status bar. Note the user can navigate to events and see XML immediately (we resolve it when they click on the event) even when this job is in progress.
• Storing these big XML strings take memory. With a set of 4 logs that caused EventLogExpert to use about 7 GB of memory before this change, we now use about 10.5 GB of memory for the same logs.

Fixes #256