Support concurrent editing of .resx files

Currently, in a LiveShare session, both the client and server can edit the .resx file in the designer. However, when one of them saves it completely overwrites the text buffer, causing the other one to reload the data--and throw away any unsaved changes.

Instead, we should preserve local unsaved changes across remote modifications to the text buffer. One way to do this would be to have the designer maintain a snapshot of the document as it was loaded as well as a snapshot of the current state. When a remote change to the text buffer is detected the local designer differ the current state against the previous state, and then apply that diff on top of the new buffer.