Prefix on post to identify synced events

[bitozoid]

This is an idea/feature.

When looking at my calendar at Google Calendar, I'm unable to distinguish between my native google calendar events and the ones synced from org-gcal.

I wonder whether a variable could be set as a prefix so events posted from org-gcal would prefix that variable to the title.

For instance, if org-gcal-post-prefix is "og: ", then an event titled

*** Go to the dentist

would be shown in Google Calendar as

og: Go to the dentist

[telotortium]

This seems like a good idea. Feel free to send a PR, otherwise I'll see when I get time to do it.