Fix for ownCloud 6.0 + interface improvements

I have fixed the problem of compatibility whit ownCloud 6.0 and made some change in css to adapt the interface for ownCloud 6. I also correct some bug with the open and close pictures in css.

Note:
You have to rename the folder of the app without char "-" to avoid the following error:
<pre> An exception occurred while executing 'INSERT INTO `oc_appconfig` ( `appid`, `configkey`, `configvalue` ) VALUES( ?, ?, ? )': SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'appid' cannot be null</pre>
Ex: "Owncloud-Files_tree" -> "files_tree"