Make a specific cache for 404 errors

This prevent the home page to send a 404 status instead of a 200.

Create a "404.error.php" file into the cache directory to store the result of a "page not found" result.

This PR can also be merged in Novius OS Dubrovka.
Prepend cache path to the 404 error prefix

to allow multiple 404 pages in cache
Add path to distinguish medias with same name in different folders

* media_path can help to distinguish 2 medias with same name in different folders
* search is now possible on media_path
New event for behaviours to alter the grid query configuration

Need some documentation if PR accepted
Merge pull request #103 from Foine/patch-5

Make a specific cache for 404 errors
Merge branch 'master/chiba2' into master/dubrovka
Merge pull request #108 from shaoshiva/patch-4

New event for behaviours to alter the grid query configuration
Merge pull request #109 from jay3/master/dubrovka

Add path to distinguish medias with same name in different folders
Merge branch 'master/dubrovka' into dev