Allow getting info or renaming part files through WebDAV

When mounting an ownCloud (backend OC) inside another ownCloud (frontend
OC), the frontend OC will use WebDAV to upload file, which will create
part files. These part files need to be accessible for the frontend OC
to rename them to the actual file name.

This fix leaves the file cache untouched but gives direct access to part
file info when requested.

This means that part file can be accessed only when their path and name
are known. These won't appear in file listtings.

Fixes #6068
Added session_keepalive setting

When session_keepalive is true (default) the heartbeat will be send as
often as the half of the session timeout value.

Backport of 912da8d to stable6
move unlink proxy to a hook which handles pre and post conditions
better error detection and don't use glob()
don't expect OC_FilesystemView, this is depreciated
added tests for the delete hooks if the trash bin is disabled
add function to extract filename from sharekey name + tests
do not show 'Add app' and 'More apps' for themed ownCloud
use appstoreenabled config switch
add icons.css file, first step to get rid of %webroot%
remove %webroot% from files app
remove %webroot from files_sharing app
remove css files from rewrite rule - there is no need to rewrite css any more
Fixed searchByMime in shared cache

- searchByMime now correctly returns files recursively search through
  all the dirs
- added unit test for searchByMime

Backport of fa5ddc3 to stable6
Merge branch 'stable6' into backport_6777_stable6

Conflicts:
	apps/files_sharing/css/public.css
	apps/files_sharing/templates/public.php