Warning needed when deleting an item

[ciar4n]

Steps to reproduce the issue

Currently items are deleted immediately after clicking the delete action (grid view). I suggest we add an 'Are you sure' warning.

Expected result

Actual result

System information (as much as possible)

Additional comments

[kasvith]

Yes, we need that, @dneukirchen :)

[laoneo]

As with the merge of #540 we have now confirmation for the context menu. Leaving this issue open as there is still the delete confirmation missing in the toolbar.