mountType attribute + delete hint fix

Added mountType attribute for files/folder to indicated whether they are regular, external or shared.

The client side then adapts the "Delete" action hint text based on this information.

Please review @icewind1991 @schiesbn @MorrisJobke @jancborchardt 

Fixes https://github.com/owncloud/core/issues/9348
I'd escape it even if uneeded here. People tend to copy code :-)