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

Pressing enter in the small quick-search-box while it was empty resulted
in a search for the current pageid.

Since we no longer change the page during a search, using the "show"
menu item to return to the current page makes more sense.

This way they can be cleanly influenced by the plugins instead of having
to modify global state.

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

That parameter was introduced in cbcc2fa5 and removed in d22b78c8

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.

The regex for the pagename lookup didn't account for `-` and `.` being
valid characters for namespaces, which lead to wrong results in the
quicksearch and pagename lookup. The full search, which already used the
queryParser, showed the correct results.

This fixes #1659

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 fixes a bug that during a search limited to `@de` would show pages
form the namespace `devel` as well (in the fullpage results).

Fixes #2285

Quick-search positioning and other templates break when removing this
tag.

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.

This makes it easier to read, reason and extend.

To allow creating links with manipulated versions of the current search
query an unparser is necessary. However, the current output of ft_queryParser
makes some advanced features hard to detect. Therefore the new
ft_queryUnparser_simple cannot handle negated phrases and `OR` searches.

It should still cover 98% of search queries.

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.