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

This was unintentionally removed in
4eab6f7c

It also should now be clearer what the code does.

This fixes #2305

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

This is useful for anonymously useable forms, like the search.

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

Implements the functionality requested in #1124

This removes a lot of reudundant CSS classes in favor of cascading
selectors. Styling isn't done, yet this is just a first step.

Still to do:

* aria labels
* styling the toggle mechanism
* RTL styling
* making sure everything displays correct when no JS is available
* translations may be missing
* refactoring the rest of search.less to make use of LESS features

This also renames the event for the search form on the search page.

The scheme for naming events that provide the new Form-class as $data is
FORM_<formname>_OUTPUT

If the entered query does not cleanly fall into the area of the given
options, show the current behavior as "custom".

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

This functionality is inspired by what other large search engines are
doing.

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.

This is the more appropriate place for that functionality, because now
it happens inside the default function for the respective pagesearch and
pagelookup events and can be properly handled by plugins.

This way we do not loose the context of the current page. Further, the
new id generated from the query before wasn't really that useful (see
issue #1124 ). And we can still generate a meaningful link "create a
page for your query", if that is considered useful.

Further redirects exist for the old urls, so bookmarked searches etc.
should mostly keep working.

This allows filtering of results by the last modified time. It still
needs a custom date entry option, highlighting of the currently selected
option (if any) and a better place where the filtering happens.

This simplifies many aspects. However, it still needs much better
styling.

This adds two new config options:

`search_limit_to_first_ns`:
Limit the search to the current X namespaces. When a search is executed
from a page within a deeper namespace, the first X namespaces will be
added as filter.
Possible use case could be with language namespaces to ensure that the
default search is initially within the current language.

`search_default_fragment_behaviour`:
Option to specify the default fragment search behavior

There are several use cases in which knowing the page where a search
request originated would be useful. This commit adds a `from` parameter
which provides that information.

Plugins may want to offer more ways to filter the search results.

Add events around each search result, both for the pagename results
and the fullpage results.

The fullpage results are wrapped in a div for better separation and
styling.
( see https://developer.mozilla.org/en-US/docs/Web/HTML/Element/dl )

These events are intended to provide plugin authors with the ability
to hydrate the search results with more information.

This add some search assistance to simple, single-word search queries
which may be restricted to a single namespace.

Further improvements:
* better styling
* trigger events for other plugins
* set namespaces directly from fulltext search results
* some more config options

Add the search form on the results page itself. This form will be used
to add more options to refine the search further.

This commit should be without functional changes.

Access attempts to security.png are normally denied, so the name of the file will be more self-descriptive