Handle readonly shared files

fixes https://github.com/owncloud/core/issues/10306 by showing a "Readonly" message and disabling the checkbox. As a result the user has the option between autorename (selecting both) or keeping the original.

![bildschirmfoto von 2014-12-08 15 30 05](https://cloud.githubusercontent.com/assets/956847/5340624/2c8d01c8-7eef-11e4-9b08-7413d8a2f440.png)

cc @PVince81 @philxtian 
remove empty else?