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
Permission granted to hijack this PR :wink: