The advanced search options list links with an invalid empty target
attribute. This fixes that by skipping empty parameters.

They are now named min and max respectively, which is hopefully less
cryptic.

As suggested by @splitbrain SearchState now behaves simliar to the PSR-7
message interfaces.

This gives create control over when a query is too complex for tool
support.

Building the new search links is complex and we're going to add another
parameter with the new sorting by mtime. Extracting a new class seems
like the cleanest way to handle that increasing complexity.