Log Message:
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