add tip about metadata driven views (and more flexible view filtering)

While writing this documentation, I realized that there needed to be a way
to stay in a view like tag=* while adding a filter like tag=work that
applies to the same field.

So, there are really two ways a view can be refined. It can have a new
"field=explicitvalue" filter added to it, which does not change the
"shape" of the view, but narrows the files it shows.
Or, it can have a new view added, which adds another level of
subdirectories.

So, added a vfilter command, which takes explicit values to add to the
filter, and rejects changes that would change the shape of the view.

And, made vadd only accept changes that change the shape of the view.

And, changed the View data type slightly; now components that can match
multiple metadata values can be visible, or not visible.

This commit was sponsored by Stelian Iancu.

debian/changelog

@@ -7,9 +7,9 @@ git-annex (5.20140211) UNRELEASED; urgency=medium
     to limit them to acting on files that have particular metadata.
   * view: New command that creates and checks out a branch that provides
     a structured view of selected metadata.
-  * vadd, vpop, vcycle: New commands for operating within views.
-  * pre-commit: Update metadata when committing changes to annexed files
-    within a view.
+  * vfilter, vadd, vpop, vcycle: New commands for operating within views.
+  * pre-commit: Update metadata when committing changes to locations
+    of annexed files within a view.
   * Add progress display for transfers to/from external special remotes.
   * Windows webapp: Can set up box.com, Amazon S3, and rsync.net remotes
   * Windows webapp: Can create repos on removable drives.