libtracker-miner: Make crawler take a maximum recursion depth

TrackerCrawler gets now the maximum depth to recurse into as a parameter
to tracker_crawler_start(), this can be used to divide crawling into
units bigger than individual folders. A value of -1 can be passed
to have it recurse with no restrictions as with the previous recurse=TRUE
argument.
libtracker-miner: Add tracker_file_system_get_file_type()

This function is useful to find out the GFileType as stored in the
TrackerFileSystem, as that's stored there from crawling time.
libtracker-miner: Store iri transiently as GFile qdata

For regular files, the data stored in TrackerFileNotifier (and
TrackerFileSystem below) will soon go away after the files are
notified through TrackerFileNotifier signals. In order to avoid
later synchronous queries to check for basic data, just preserve
it as GObject qdata.

This reduces the number of queries performed at the time of
processing those files. Especially noticeable on ::file-created
events, as these queries were done regardless of Tracker knowing
these files are brand new...
libtracker-miner: Perform leveled notification in TrackerFileNotifier

The current notification process involves crawling over index roots
without restrictions, and querying the state of every file in the
store. This is fastest, but can get memory hungry on huge directory
trees.

So split the process in 3 sequencial steps, that are repeated from
top to bottom over the directory hierarchy:

- A directory is crawled, contents that currently exist in the
  filesystem are extracted.
- Only if the directory is an index root, or was checked to exist
  in the store through previous iterations, the directory and all
  contents found are looked up on the store by their uri, new and
  updated contents are detected by comparing mtimes.
- Only if the directory passed #2, and its mtime changed (which
  usually implies something was added or removed, at this stage we
  only have to care of the latter), query all elements in the store
  that nfo:belongsToContainer to it, and check for those files that
  existed in the store but don't exist anymore. Deleted contents
  are detected in this stage.

The change has been done so there is certain compile-time granularity
on the directory processing, currently controlled through the MAX_DEPTH
define. This switch controls the maximum depth on crawled/queried chunks,
which establishes some indirect limit on the number of GFiles (and all
misc data around) that are in memory at the same time.

From testing, first-time crawling performance is completely unaffected,
and second-time crawling on an unchanged directory tree has negligible
decreases. The IN() match on an indexed property like nie:url looks
near constant, and the third more expensive step will only happen
when it is very likely that there are actual changes to process. So
the MAX_DEPTH value has been set to 1 to keep memory usage to a minimum
(tracker-miner-fs now peaks on massif at 24MB when it previously early
grew to ~180MB, indexing 11304 folders and 123428 files)
Merge pull request #26 from pvanhoof/subtree-crawling-tryout

Subtree crawling tryout