Deprecated Workflow Dropbox Module

The workflow dropbox module seems to have been designed to configure a workflow class
with some arguments, and so the built-in
org.onehippo.cms7.repository.wfdropbox.DropboxDaemonModule queries all
the nodes having 'wfdropbox:call' mixin type and invoke the configured
workflow method with the arguments. Finally, it removes the mixin type
not to invoke it any more. So, it is intended to invoke only once. If
you want to execute something periodically, then you have to register a
scheduler by yourself. (See
CheckBrokenLinksWorkflowImpl#checkLinks(CronExpression))

It's similar to DaemonModule feature, The only difference is you should
configure a workflow class, but I don't think it would make any more
conveniences now.

Since 7.8, we can easily configure a daemon module. (We can set only
repository module configuration now; we don't have to do it in
MANIFEST.MF any more. [2][3]) And we can easily register a scheduled
task [4][5] in a module.

It seems a lot easier to follow [2] than wfdropbox:* stuffs if I want to
get some components invoked when repository starts.

So, we should deprecate the wfdropbox feature,
which doesn't seem to have been used elsewhere than broken links to me.
Broken links checker should be improved not to use it.

[1]
http://www.onehippo.org/7_8/library/concepts/editor-interface/configure-the-broken-links-report-add-on.html
[2]
http://www.onehippo.org/7_8/library/concepts/hippo-services/repository-managed-components.html
[3] https://issues.onehippo.com/browse/REPO-624
[4]
http://www.onehippo.org/7_8/library/concepts/hippo-services/repository-scheduler.html
[5] https://issues.onehippo.com/browse/REPO-620