shorter names are more in line with what we already use, makes less
layout problems in the documentation and is easier to type when telling
people about it.

This should make their purpose more obvious.

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

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

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

This also adds the default theme color for the DokuWiki template as the
green from the existing links.

The goal is to make the css replacement accessible in other contexts,
for example for the manifest.