itemsPerPage/maxItemsPerPage: Support Collections and split

[m-mohr]

Currently itemsPerPage applies to search (collections + items), items, but not collections.
maxItemsPerPage applies only to search.

This is not consistent and fine-granular enough.

I'd propose to split into the following options:

• searchResultsPerPage - for search within collections + items (default: 12 as before)
• itemsPerPage - for items (default: null, use server default)
• collectionsPerPage - for collections (default: null, use server default)

Rename maxItemsPerPage to maxSearchResultsPerPage.