search-directory: enable incremental directory load

Instead of emitting nautilus_directory_done_loading() from the
"finished" callback, do it as soon as we receive the first batch of
hits. The rest of the results will be added to the directory in
background.

https://bugzilla.gnome.org/show_bug.cgi?id=663242
search-directory: always start the simple engine

This is needed to load results incrementally, which will be done in a
separate commit.

https://bugzilla.gnome.org/show_bug.cgi?id=325146
search-directory: make the simple engine recursive

https://bugzilla.gnome.org/show_bug.cgi?id=325146
search-directory: ensure we call emit_done_loading() when no results

When we receive no results, we should still call
nautilus_directory_emit_done_loading() and clear the loading state.
Do this by factoring out an idempotent helper, which will be called both
from the hits-added and finished callbacks.
query-editor: don't quote the current location name

We don't do it anywhere else in the UI for buttons that only display the
location name, and it looks strange.

https://bugzilla.gnome.org/show_bug.cgi?id=687869
search-engine-simple: make sure to query for IS_BACKUP attribute

Or we will fail to filter out backup files from search results.
search-hit: fix mem leak

https://bugzilla.gnome.org/show_bug.cgi?id=691004
search-hit: plug a memleak
shell-search-provider: plug a memleak
search: print a warning when a provider errors out on a query
tracker: fix an error with latest Tracker

We name some query column variable results with "AS ?foo", but Tracker
now errors out on those clauses, not returning any result.
Since we don't need them, just remove them from the query string.

https://bugzilla.gnome.org/show_bug.cgi?id=693195
search-engine-simple: don't follow symlinks when descending

It's inconsistent with "find" and it can yield to unexpectedly large
result sets.
Do not follow symlinks when recursing the search down the hierarchy.

https://bugzilla.gnome.org/show_bug.cgi?id=697181
search-hit: add a missing break
query: rank prefix matches lower than exact matches

Count how many letters are left after the string occurrence, and
subtract that from the match score.
file: do not reverse collated order for files with same search rank

When two search results rank equally, we fall back to the collated
order.
Since search is usually ranked reversed (highest at the top), we need to
ensure we don't accidentally reverse the collated order as well.

https://bugzilla.gnome.org/show_bug.cgi?id=688772