magit-hunk-set-window-start: cosmetics
rename magit-find-section to magit-get-section
magit-section-region-siblings: don't use magit-find-section-at

We don't need to fallback to the `magit-root-section' when at
eob in this function, and (get-text-property POS 'magit-section)
is more expressive.
refactor section movement

Add new section movement commands
  magit-section-forward
  magit-section-backward
  magit-section-forward-sibling
  magit-section-backward-sibling
  magit-section-up
replacing
  magit-goto-next-section
  magit-goto-previous-section
  magit-goto-next-sibling-section
  magit-goto-previous-sibling-section
  magit-goto-parent-section
and the utility functions they used
  magit-find-section-after
  magit-find-section-after*
  magit-find-section-before
  magit-find-section-before*
  magit-find-section-at

The new implementation is more straight-forward and handles edge-cases
better.
refactor section visibility toggling

Add new section visibility commands
  magit-section-show
  magit-section-hide
  magit-section-toggle
  magit-section-toggle-children
  magit-section-show-children
  magit-section-show-headings
  magit-section-cycle
  magit-section-cycle-global
  magit-section-cycle-diffs
and utilities
  magit-section-show-children-1
  magit-section-show-headings-1
  magit-section-hidden-body
replacing commands
  magit-show-section
  magit-hide-section
  magit-collapse-section
  magit-expand-section
  magit-toggle-file-section
  magit-toggle-section
  magit-expand-collapse-section
  magit-cycle-section
and utilities
  magit-section-set-hidden
  magit-section-any-hidden
  magit-section-collapse
  magit-section-expand
  magit-section-expand-all-aux
  magit-section-expand-all
  magit-section-hideshow
Also refactor
  magit-show-level
dropping
  magit-show-level*
  magit-section-lineage

The new implementation is inspired by Outline and Org-mode, to some
extend.  It has more features than the old implementation and fixes
several bugs.
Merge branch 'n/sections' into next