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

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

This function is now deprecated.

Tests even surfaced some smaller bugs :-)

Othewise weird side effects may happen when a parent constructor
overwrites them again.

getLink() now no longer escapes ampersands - this is task of the code
producing the HTML. When using asHtmlLink(), buildAttributes() will take
care of the escaping for you.

This basically reimplements tpl_action_dropdown but makes use of the
other menus (by setting the context to mobile) and thus still triggers
the appropriate events. This makes #1598 obsolete.

tpl_action_dropdown (and the other tpl_action* methods) still need to be
replaced to make use of the new Menu system.

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

also adjusts styles and adds images

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.