Add "watch file for changes" support for back-ends supporting it

[munen]

The Dropbox JS API has a feature to watch a file for changes:

• https://dropbox.github.io/dropbox-sdk-js/Dropbox.html#filesListFolderLongpoll__anchor
• https://www.dropbox.com/developers/documentation/http/documentation#files-list_folder-longpoll

This feature could come in especially handy when organice starts to allow for multiple files to be opened while staying efficient in terms in traffic and latency. It's not a requirement, at all, it would just come in handy to have a higher certainty of ensuring data consistency across clients.

[munen]

@tarnung I'm adding this feature to the 'multi-file support' project, but it's not necessary at all to add it at this point, of course. The rationale is that it would already be helpful if people use multiple clients or share their single Org file across multiple people. The impact would just increase when people start using organice with multiple files open at the same time.

However, this issue certainly has no priority and would likely - if at all - be added as the last.