Background fetch plugin

This plugin provides support for periodic background fetch of all remotes. A lot of people work with quite a few remotes and a few repositories open at the same time, and having this integrated into GitExtensions allows easily configuring a periodic fetch of all remote changes, so we don't have to wait for the UI afterwards.

I did notice that there doesn't seem to be any notification for the plugins when the application is shutting down. It might be interesting to call SetWorkingDir("") when the application window is closed so that plugins have a chance to cleanup.