Listen for external changes using FileSystemWatcher

[SLaks]

For example, git pulling a new settings file should apply without re-opening the solution.

This would also automatically synchronize global settings between VS instances, and across computers if the user settings file is in a cloud storage provider (see #3). (just like VS 2013's settings sync, but for older versions of VS, and across versions, too)

This should also catch when a new settings file is created for the solution (eg, via git pull), which may be more complicated.