placesview: simplify code

placesview: simplify code

Since we started adding a persistent Computer item,
all the code related to the view modes became obsolete,
since the view is never empty anymore. So, drop this
dead code and use the plain stack to manage the empty
search results view.

Also, this patch fixed a very annoying keyboard navigation
issue where we couldn't go from the On This Computer to Networks
lists, because they were two separate widgets. Merge the two
lists into a single one, and update headers accordingly.