Provide support to synchronize settings across machines

Problem

There are quiet some extensions out there to handle setting synchronization between multiple computers. And in my opinion they are very important: Since vscode is very customizable with settings, keyboardshortcuts and extension it is very hard to synchronize those things between multiple workstations. And all those extensions are not really clean solutions, because vscode does nearly nothing to support them (for example the paths vscode uses are not exposed, see #2741). So here some ideas to make synchronization clean and extensible: - add a contribution point for synchronization extensions - Those extensions should not have to gather all the files that have to be synced. vscode should do this and then pass them to the extensions - Such an extension's job is only to upload/download the files, passed to them (so we can have the `gist upload`/`svn upload`/`git repo upload`/`dropbox`/`filesystem`/`onedrive`/`ftp`/`other services`. - Or maybe gather all settings into one big json object and pass that. - Support versioning and non versioning extensions - add a remote_settings.json to build on top of the workspace-settings/user-settings stack. - allow automatic and user triggered synchronization Ok... just some ideas :smile: