tex-tcolorbox{,-doc}: update to 4.15

- bug fix: 'tikz upper', 'tikz lower' failed to take more than
  one parameter (again)

- bug fix: 'tcboxedraster' could not be set to be 'breakable'

- library 'breakable': Implementation of 'pad after break' changed to
  be consistent with a boxed title.

- library 'raster':
  * dimension settings changed to be more robust
  * new options 'raster width center', 'raster width flush left',
    'raster width flush right'

- Example source code of 'tcolorbox-example-poster' and 'tcolorbox-tutorial-poster'
  adapted to be compatible with lipsum package version 2.0

(markd)