More documentation improves

* Update date and version numbers
* Use suitable tags, add empty lines and re-indent (more readable)
* Suitable capitalization and punctuation
  - Preserve upercase capitalization in the 'Practice Mode' section
* Text added to 'screeninfo' and 'caption' tags (screenshots)
* Add guiicons an use more entities
* GUI sync and shortcut fixes
* Delete a section:
  - Counting an Answer as Right
    <!--FIXME the icons are not displayed in parley > 15.12 -->
  - Parley_practice_icon_right.png
  - Parley_practice_icon_wrong.png
* More Screenshots deleted:
  - configure_parley.png
    Used superfluously to reflect a menu item use "Settings -> Configure Parley..."
  - new_lesson1.png
    Used superfluously to reflect a context menu use (Add the three ways to do it)