[feature request] Automatic sync on quitting the app

[d1vanov]

It would be useful to have an option to automatically run sync before quitting the app if the current account is Evernote one, so that propagating the locally made changes to Evernote is not forgotten.

However, need to differentiate explicit quit against signal sent by the OS on shutdown. It's not a good idea to slow down the OS shutdown by trying to run sync.