DocumentsUI: Add a standalone File Manager

This commit adds a standalone mode to the DocumentsUI, to let
it act as a file manager/explorer.
You can browse your storage contents, open files, and delete them,
as well as do everything that the DocumentsUI can do (ie. sort
by name/size/type, list and grid view, recents, search, ...).

This gives us a consistent file browsing UX accross all apps
that uses the new documents API.

Change-Id: If73a3c100f010bdb766c843976d4fd3573ea805c
trds: improve dialogs and menu popups

Change-Id: I54da4ac0236dfe16ee964ccb070944206d0544e2
trds: consistent glow for menu dropdown and popup

Change-Id: I248872cbd67471495f8b95036d995f25e2ca4459