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

This makes it more flexible to use on the expense of needing one echo
more.

* use inline-block instead of flexbox
* fix RTL alignments

This introduces a new dokuwiki\Ui namespace and refactors the Admin
screen into a Ui class. The ultimate goal is to split up the big,
complex functions in inc\html.php in better maintainable classes in the
Ui namespace. This is the first go at it. Others function->class
conversions should follow.

This also switches the icons for our base admin plugins to inline SVG.
(files and styling not included, yet).