When deleting spaces that contain hidden pages warn the user about it

Right now by default the Admin user has 'Display Hidden Documents - No'.

Looking at the Document Index he will not see any pages displayed inside the space so he can decide to delete the space without knowing that it contains hidden pages and this can have a ulterior effect.

An example is XWIKI-7930. After another Admin (that has 'display hidden - yes') is using the application, a page is created by the application. As a default Admin (with 'display hidden - no') the index will not display other pages inside that space and the created page is empty (and can look as a test), so he may decide to delete the space (since it has the rights).

A warning that the spaces contains hidden pages would help.