Indicator to show if you have the latest version of Plinth

This was mentioned in the last progress call (https://wiki.debian.org/FreedomBox/ProgressCalls/2016-12-18).

After a Plinth release is announced, it's not clear when it will show up in unstable / testing / stable. So users might expect an update to happen, but it's not available for them yet.

In Help -> About, we show the current Plinth version. We can also have an indicator to show if a newer version of Plinth is available, in the configured apt source.