Include the indent level in result from backend

Part of #340.
Add indent as filter for XHR search

The XHR version of smart_search now allows filtering based on indent
level.

Part of #340.
List top level prefixes on empty search

When performing an "empty search", ie with a zero-length search string,
only list the top level prefixes (indent = 0). An empty search is
probably a sign of that the user wants to see everything and showing
only top level prefixes is probably a better starting point to give an
overview than directly listing and expanding all prefixes.

Fixes #340.
Merge branch '340-list-top-level'