Feature request: check whether dropbox is up to date

Recently I installed dropbox via the `nautilus-dropbox` apt package, and with the provided command line interface.

I experienced, that `dropbox update` does download the latest version regardless of the currently installed version.
This means, if I run the command twice in a row, it will do a download twice.

I would like a method to check the current (online) available version.

For example, `dropbox update` would only update, if a newer version is available, or if a new `--force` flag was passed.

Maybe the `version` command would output an extra line like `update available`.
```
> dropbox version
Dropbox daemon version: 163.4.5456 (Update available)
Dropbox command-line interface version: 2019.02.14
```

Or a separate command, like `dropbox check-update` that would:
* output `Up to date` and return 0 if it is up-to-date and
* output `Update available` and return 1 if it is not.

These are ideas, should be revised to ensure the CLI is consistent with itself.

Start by tracing the CLI implementation of the `dropbox update` and `dropbox version` commands. Compare how the current daemon version is obtained with the requested online-version check, then define one consistent command or flag behavior and verify its exit status and repeated-update behavior.

Written by the indexing model from the issue text.