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.
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