Docviewer should not look recursively for documents, but should allow to browse into a single folder

HOW TO REPRODUCE:
1) Open DocViewer

WHAT HAPPENS:
* A list including all the supported documents from ~/Documents and $[EXT_MEDIA]/Documents is shown.
* It is only allow to sort files by size, name, date, etc.

WHAT SHOULD HAPPEN:
* Only recently opened files are listed by default.
* The list of recent docs can also be filtered by document type or storage (Internal storage, SD-card, etc.)

* By tapping on a "Open/Import" action in the header, a file picker is shown, and the user can browse through the file system folders.
* DocViewer should provide a list of places which is "safe" to browse: ~/Documents, ~/Downloads (since webbrowser-app can now download files by itself), and all the removable storages plugged into the system (including SD-card, hard-disk or USB pen drives).

* A document loaded through the picker will be available as a recent file for a next iteration.

This would match the behaviour of the MS-Office apps for the Android platform[1], and would be suitable for a convergent environment when Unity8 will run on desktops too.

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This has been informally discussed some week ago during one of the weekly DocViewer meeting.
In that occasion, it has been told that a platform implementation of a File Picker has been discussed internally in the UX team.

Although the solution I proposed above requires the DocViewer to implement a file picker, I guess this should be provided by ContentHub.
An application should only have to implement a ContentPeerPicker[2] which, in addition to the list of applications/peers that handles the specific content type, should provide a new section with a list of places/storages where to pick a file from.
Additionally, it may be allowed to access to root after a PAM authentication (as done for filemanager-app and terminal-app).

This was the first implementation of file picking done for DocViewer, but at that time it has been deprecated because of its dependency on filemanager-app (which is not preinstalled on Ubuntu devices).

[1] https://play.google.com/store/apps/details?id=com.microsoft.office.word&hl=it

[2] https://developer.ubuntu.com/api/apps/qml/sdk-15.04.1/Ubuntu.Content.ContentPeerPicker/