Better organize Panel topbar menu

The topbar menu of Kirby looks good and clear when not further extended (like in a starterkit). But when it gets extended it can become unintuitive.

I suggest:

1. Place non-default entries (plugin views) in a separate section
   and group “Users” with “Your account” (see screenshot).
   Currently the default menu entries and plugin views get sorted alphabetically and therefore mixed up.
   • Optionally the entries could be configurable and sortable
     (e.g. in the site blueprint or config).
2. The “Settings” label is unclear
   • should be called “System” as described in #90
     (or configurable in the site blueprint or config)
   • and/or extendable as described in #63 and #230
3. Languages could get an own menu entry/view when activated
   to make it more clear for editors/translators