Tray menu: Clarify "Up to date" item with time of last E-Tag check

[guruz]

When I wake up my laptop while being on flaky WiFi I don't know to what exact time the "Up to date" confirms to.

[ckamm]

@guruz Sounds good. You suggest a message like "Up to date as of 2017-3-24 11:59"? Possibly eliding the date if it is the same as today.

This would also mean expanding on the states this part of the tray menu can have. Currently we display "Up to date" whenever no sync is running.

Possible suggestion:

• The oldest etag reply over all non-paused folders is x, display "Up to date as of {x}"
• What about network errors either for etags or for account connectivity? Display "Network errors"?
• What about signed-out accounts? Display "Some accounts are signed out".

A 4-word summary of the status of several accounts with multiple folders each is tricky.

[TheOneRing]

Probably https://github.com/owncloud/client/blob/master/src/gui/folder.h#L452 is a good source.