Menu: check context visibility after assembling items

This addresses #4400. By checking the context visibility after the
assembly, the context is properly checked for plugin items.

code style: line breaks

codestyle adjustments: class declaration braces

Apply rector fixes to inc/Menu

deprecated trigger_event() in favor of a static method on Event

introduce a MenuInterface

this helps tieing together the "normal" menus and the MobileMenu and
makes them both implement a getItems() method.

used index array for menu items

This makes it a little easier to splice new elements into position

more doc blocks for the menu system

use menu for site tools

this adds an option to print the menu without icons (as we do in the
dokuwiki template) but icons were added nontheless

fix base dir for images

beginning of a complete refactoring of the menues

This is an attempt to:

* get rid of the super long, complex functions in in/template.php
* make it easy for plugin authors to add their own items t

beginning of a complete refactoring of the menues

This is an attempt to:

* get rid of the super long, complex functions in in/template.php
* make it easy for plugin authors to add their own items to any
of our menus, regardless of the used template
* continue the progress to make use of SVG in the menues

This takes a similar approach as my actionrefactor branch. Originially
I thought both refactorings could be done in one, merging the
functionality of DokuWiki actions and the menu items. However I couldn't
make it work. So instead we have two separate but similar things.
Maybe they can converge later on.

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